Camera won't take photos, battery is fully charged, I performed a reset using wrench, there is a red circle in the viewfinder or it projects a red light from front of camera but will not take pictures, leaving on vaacation in a few days of course, 2nd major issue I've had with this camera, hate it

The Display is OLED and when it is off it is really OFF.
FYI:
This camera has FIVE (5) Brightness levels for the display. The Brightness is usually set in the TOOL (wrench) menu.

If you are positive the batteries are good, fresh and or fully charged it is MY OPINION that the camera is non functional. The Viewfinder is critical for camera operation. Sorry I am not much help here it seems this camera needs either repair or replacement.

Your camera can work with 2 AA sized batteries.
Always charge NiMH batteries before using them for the first time, or if they have not been used for a long period.
Always charge sets of batteries together.
Do not use alkaline batteries unless it is absolutely necessary. In most cases, alkaline batteries may have a shorter service life than NiMH. Alkaline battery performance is limited, especially at low temperatures. The use of NiMH batteries is recommended.
AA manganese (Zinc-Carbon) batteries and NiCd rechargeable batteries cannot be used with this camera.
If you are sure you have good batteries, also check if all contacts are clean and shiny. No oxidation is allowed. To clean contacts never use sharp tools. They can damage the camera forever.
Eventually clean contacts with a cotton swap or a dry cloth.
Also check if no debris is in the battery compartment. Just a little pice of paper can spoil all the (camera) fun.
Just to make sure, try some other batteries, then the ones you are sure of, they are fully charged. Perhaps, next time you are less convinced.

How long you have the battery? I suggest you to fully discharged the battery and then charge it as much as you can, do this circle for several times. Hope this can help

Try to load Nikon Camera Control Pro 2 from Nikon sites.
It\'s free for 30 days.

try leaving the camera on to drain the battery down, then try charging it again.. or could be a bad battery

First, the "r06" message is NOT an error code. It indicates the number of shots remaining in the internal memory buffer, before the camera must prevent additional shutter actuations so that it may transfer those images already in the buffer to the memory card. This "r" number will decrease with each shot taken in rapid succession until it reaches 0. The shutter release will no longer work until the buffer is transferred to the memory card, then shooting may continue again.

As far as not being able to even see through the viewfinder, it sounds like something may be obstructing the light from reaching the meter and viewfinder. The problem is most likely the position of mirror inside the camera body. With the lens off the body, the mirror should be plainly visible at about a 45 degree angle to the opening. A side view drawing of this is below. The solid red line is the mirror in the normal position. The red dashed line is the up position of the mirror when the shutter is released.



When the mirror is in the "normal" position, the light from the lens is projected on a screen so that the image is visible in the viewfinder for composing and can be metered. When in the "up" position (when the shutter release is fully depressed), the light from the lens is projected on the camera's sensor for as long as set by the manual settings or program; based on ISO, aperture, etc. At the end of this time, the mirror returns to the "normal" position.

If your D100's mirror is not in the lowered 45 degree angle position, the image seen in the viewfinder is inside the camera - not that which the lens would project. Hence, the dark viewfinder, long exposure times and - I'm guessing - severely overexposed pictures because way too much light is striking the sensor because the meter is only seeing darkness.

If the mirror is ok, with the lens removed from the body, look for the aperture lever as shown in the yellow circle in the picture below:



By default, the aperture is at minimum. The camera moves the lever from this position to full open (and anywhere between) as needed. You should gently move the lever to the other end of its travel to open the aperture to maximum. Point the lens away from a light source but preferably at a light colored background. Look through the lens. There should be no obstructions and be clear. Next, look at a dark colored background to find the same results. If you want, you can even allow sunlight to shine through the lens onto a sheet of paper (like a magnifying glass). The result should be a bright circle with no obstructions. Obstructions in the lens will prevent the meter in the camera from getting accurate information about the scene and if significant enough, prevent viewing through the viewfinder.

You may wish to have the camera & lens professionally cleaned and serviced to repair a mirror or lens issue.

If this was helpful, please rate it as such. Good luck!

Solution: As digital cameras become more advanced and include more features, the power needed to keep them functioning has also increased. That means that typical alkaline batteries likely won’t do more than power your camera for a few shots before running out of juice. Sometimes, your digital camera may not power on at all if the inserted alkaline batteries aren’t fully charged or powerful enough. Rather than carry along dozens of alkaline batteries for a single photo shoot, you can save money by purchasing rechargeable NiMH (nickel-metal hydride) batteries. Rechargeable batteries can be purchased with a charger that you plug into a wall outlet, and they fully charge in one to five hours depending on the charger and the type of batteries you’re using.

If you’re already using rechargeable batteries but are experiencing short battery life with your digital camera, it’s likely time to replace your rechargeable batteries with new ones. Depending on the number of charging cycles you perform (how many times you discharge and recharge your batteries), rechargeable batteries typically last for two to three years before they no longer hold a full charge. You may also ensure that you’re charging your rechargeable batteries directly before use. As batteries sit unused for extended periods of time, they automatically lose some of their charge.

There are many ways you can conserve battery life while using your digital camera, too. For starters, if your camera has a viewfinder in addition to an LCD, use the viewfinder to frame your shots and turn the LCD off because LCDs draw a lot of power. If you prefer to use the LCD, you can still conserve battery life if you refrain from looking at each picture on the screen after you take it.

Solution: As digital cameras become more advanced and include more features, the power needed to keep them functioning has also increased. That means that typical alkaline batteries likely won’t do more than power your camera for a few shots before running out of juice. Sometimes, your digital camera may not power on at all if the inserted alkaline batteries aren’t fully charged or powerful enough. Rather than carry along dozens of alkaline batteries for a single photo shoot, you can save money by purchasing rechargeable NiMH (nickel-metal hydride) batteries. Rechargeable batteries can be purchased with a charger that you plug into a wall outlet, and they fully charge in one to five hours depending on the charger and the type of batteries you’re using.

If you’re already using rechargeable batteries but are experiencing short battery life with your digital camera, it’s likely time to replace your rechargeable batteries with new ones. Depending on the number of charging cycles you perform (how many times you discharge and recharge your batteries), rechargeable batteries typically last for two to three years before they no longer hold a full charge. You may also ensure that you’re charging your rechargeable batteries directly before use. As batteries sit unused for extended periods of time, they automatically lose some of their charge.

There are many ways you can conserve battery life while using your digital camera, too. For starters, if your camera has a viewfinder in addition to an LCD, use the viewfinder to frame your shots and turn the LCD off because LCDs draw a lot of power. If you prefer to use the LCD, you can still conserve battery life if you refrain from looking at each picture on the screen after you take it.

Reset the menus to default or factory settings.
Then test the camera again.
Make sure that the battery is fully charged or new.
You might consider to buy a new battery (Ebay is a good place) for this camera. Then test.
Problems like this can show up when the battery is weak or not fully charged.
You may try to remove the battery for 24 hours and then after try it again.

It sounds to me like you are not waiting for the camera to charge the flash when turning the camera on or between shots. It seems you are using 1700mAh batteries and most modern digital cameras are better off using 2500mAh batteries. Your camera was released in September 2006 so it is considered "modern". With 1700mAh batteries, your flash (warm-up) recycle time time will be rather long or the amperage may not be enough to fully charge the flash unit. I highly recommend Duracell 2500mAh rechargeable batteries.