Error blink in place of shutter indicator

This probably indicates that the selected shutter speed is too slow for hand-holding, and any images may suffer from camera shake. You can increase the shutter speed (maybe by selecting a higher ISO setting), use a support (a monopod or a tripod), or use flash.

Check the batteries

Be sure your mode dial is not in the auto position. If it is, the camera will only function with the lense also locked to the auto position, which will either be the lowest aperture (highest number) or sometimes an "A" setting separate from the normal aperture settings. That's generally what this type of indication means on program/manual models - that you're trying to use manual settings, when the camera is set up for auto program exposure.

if this is a slr camera ensure the lens is fitted correctly or that the connectors on the lens and body of camera are clean.

also try changing setting out of AE "apature priority" to another setting such as auto, the indication could mean that the place where the photo is being taken is too dark and that the apature setting for the speed set is too small for a decent image. and the apatures needs to be open further or the shutter speed needs to be slower or the IOS/ASA needs to be raised. to stop bluring at slower shutter speeds or in dark areas it is advised to use a tripod or other sability device.

I hope this was useful.

Try shooting in manual mode with lens set at maximum (wide open) aperture. Then try at a smaller f-stop, say 5.6 or 8. If the error message appears when your lens is set to the smaller aperture, you could have a lens problem. Try another lens using the same test. If a known good lens gives an error message, then the problem is in the body. Also try zooming to different focal lengths and see if your 18-105mm gives the error at particular focal lengths. An error here would indicate a lens problem. In any case, your lens or body would have to be repaired by a professional as these repairs require major disassembly and re-calibration.

This is an indication that the camera is not "talking" to the lens correctly. When you press the shutter button, the camera tries to focus, set light levels, and then shoot the picture. If it can't focus or set F-stop and shutter speed, then it can't shoot the picture and gives you the error indication. SO
1. You need to make sure the lens is an automatic lens capable of working with the D90. 2. You can take the lens off and back onto the camera a few times in case its a dirty contact issue or just didn't seat correctly. 3. Set the D90 to manual and make sure you move the AF selector on the front to M. Then you would manually adjust all the settings. That should take the auto-coupling out of the picture (pun intended)
4. If it still shows error when in total manual mode, there may be something wrong in the camera itself. Before giving up, do a total system reset on the D90 (I assume you have the manual).

I've also got a D90- nice camera. I only get the err message when the lens is not seated correctly or the f-stop ring is not set to the max. If you check the settings and assembly correctly and they seem OK, then the contacts may be dirty or faulty. If the indicator is blinking pre-shutter release, that may indicate the same issues. If you have another lense to try, that may be a good indicator of bad contacts or issues in the lense.

Besides the auto-flash mode, you can select fill flash, flash off, self-timer, and nighttime-view with or without flash.
Press the MODE-SELECTOR BUTTON until the desired indicator appears on the LCD PANEL.
Fill flash and flash off modes remain selected after you take the picture. To cancel the selection, press the mode-selection button or close and reopen the lens cover/flash to return to the auto-flash mode.
Fill flash
When bright light indoors or outdoors comes from behind the subject (backlighting), dark shadows (especially on faces) may occur. Use fill flash to lighten these shadows.

1. Open the LENS COVER/FLASH to raise the flash and to turn the camera ON.
2. Press the MODE-SELECTOR BUTTON repeatedly until the FILL-FLASH INDICATOR appears on the LCD PANEL.
3. Frame your subject within the VIEWFINDER EYEPIECE.
4. Partially depress the SHUTTER BUTTON. When the FLASH/CAMERA-READY LAMP turns off and the fill-flash indicator on the LCD panel stops blinking, fully depress the shutter button to take the picture.

Flash off
When you do not want to use the flash, especially indoors where flash is prohibited, such as in theaters and museums, or when you want to take twilight scenes or distant subjects that are beyond the flash range, or capture the ambiance of existing light, use the flash-off feature. Use a tripod or place the camera on another firm support, and use high-speed film because the shutter speed in these situations will most likely be slow.

1. Open the LENS COVER/FLASH to raise the flash and to turn the camera ON.
2. Press the MODE-SELECTOR BUTTON repeatedly until the FLASH-OFF INDICATOR appears on the LCD PANEL.
3. Frame your subject within the VIEWFINDER EYEPIECE.
4. Press the SHUTTER BUTTON to take the picture.

Self-timer with auto flash
Use this feature to include yourself in pictures.

1. Use the TRIPOD SOCKET to attach the camera to a tripod or place it on another firm support.
2. Open the LENS COVER/FLASH to raise the flash and to turn the camera ON.
3. Press the MODE-SELECTOR BUTTON repeatedly until the SELF-TIMER INDICATOR appears on the LCD PANEL.
4. Frame your subject within the VIEWFINDER EYEPIECE.
5. Press the SHUTTER BUTTON. The SELF-TIMER LAMP will glow and then blink during the last three seconds of countdown.
   • To cancel the self-timer selection before the shutter releases, close the flash.
   • The self-timer automatically turns off after the shutter releases.

Night-view with flash
In this mode, the camera balances the flash and existing light exposure so you can take beautiful pictures of people at sunset or at night. Use a tripod or place the camera on another firm support, and use high-speed film because the shutter speed at night will most likely be slow.

1. Open the LENS COVER/FLASH to raise the flash and to turn the camera ON.
2. Press the MODE-SELECTOR BUTTON repeatedly until the NIGHTTIME-VIEW INDICATOR with flash appears on the LCD PANEL.
3. Frame your picture in the VIEWFINDER EYEPIECE.
4. Press and hold the SHUTTER BUTTON for 12 seconds without lifting your finger. Note: If you remove your finger from the shutter button before 12 seconds, the shutter will time out immediately.
   
5. After 12 seconds, release the shutter button to take the picture.

Night-view without flas
In this mode, you can capture the natural existing light of city-night scenes or fireworks at night. Use a tripod or place the camera on another firm support, and use high-speed film because the shutter speed at night will most likely be slow.

1. Open the LENS COVER/FLASH to raise the flash and to turn the camera ON.
2. Press the MODE-SELECTOR BUTTON repeatedly until the NIGHTTIME-VIEW INDICATOR without flash appears on the LCD PANEL.
3. Frame your picture in the VIEWFINDER EYEPIECE.
4. Press and hold the SHUTTER BUTTON for 12 seconds without lifting your finger. Note: If you remove your finger from the shutter button before 12 seconds, the shutter will time out immediately.
   
5. After 12 seconds, release the shutter button to take the picture.

OM-20 was basically a upgraded OM-10 with the manual adapter built in and a number of other refinements.

The viewfinder has LED's to show the shutter speed recommended by the camera's lightmeter for the ISO and aperture selected. It also has an exposure compensation indicator (the +/- symbol) and an indicator for flash ready which doubles up as a post-exposure flash confirmation. There is also the indicator lamp to show manual mode has been selected. OM-10 lacks the manual mode lamp and the +/- indicator.

Like the OM-10, the OM-20 is primarily an aperture priority automatic camera. In this mode you set the ISO film speed, choose which aperture you wish to use (with the ability to use the lens depth of field preview button) and then the camera selects the correct shutter speed. The +/- exposure compensation control allows the user to tell the camera to modify the recommended shutter speed by up to two stops either way.

In manual mode, there is no manual metering. The light meter behaves exactly as it does in aperture priority mode and the viewfinder shows the recommended shutter speed and not the manually selected one. Correct metering is therefore a case of adjusting the aperture first, and then choosing the correct shutter speed indicated in the viewfinder. If the user then decides to select a different shutter speed, then the aperture ring must be adjusted to maintain the correct exposure. For example the aperture is set to f8 and the camera recommends 1/60th of a second. The user decides that a faster shutter speed is required and chooses 1/250th, but the viewfinder remains showing 1/60th. In order to keep the same exposure value the user must open the aperture by two full stops to f4. The camera's light meter will detect the new aperture setting and providing the light on the object is unchanged the viewfinder shutter speed display should now show 1/250th as well to confirm the correct adjustment. Alternatively, the user can choose the shutter speed first by looking at what has been set on the control ring (or by turning the ring to the end of its travel and then counting the clicks from there as all experienced OM users do) and then turning the aperture ring until the shutter speed shown in the viewfinder matches what's been manually set.

It all sounds clumsy and complex but is done far more quickly than I've taken to type this and becomes second nature.

Aperture priority metering is selected on the camera by choosing AUTO on the mode selecter. In this mode the shutter speed ring has no effect and the viewfinder always displays the automatically selected shutter speed.

The shutter button features a two-stage action. Pressing Halfway Pressing halfway automatically sets the exposure, focus and white balance.
The indicators light or blink as follows.
Upper Indicator
" Green: Metering complete (two beeps sound)
" Orange: Flash will fire
" Blinking Orange: Camera shake warning / Insufficient exposure
Lower Indicator
" Yellow: Macro Mode / Manual Focus
" Blinking Yellow: Focusing difficulty* (single beep sounds)
* When the indicator blinks yellow, lock the focus before shooting.