Hi, my canon 300d gives

err 99 is moisture or dew inside the camera.

Error 99: tthe lens contacts are dirty- get a pencil with an eraser on the end, just a regular old pencil with a red rubber eraser. Detach the lens from the camera, hold it so that the lens’ gold contacts are pointing down and lightly erase their exposed surface, cleaning them of any hand oil that might have gotten on them. Do the same thing with the gold contacts inside the camera body. This is a bit harder but it’s doable: just hold camera so lens opening points down so no gunk falls in. Erase lightly. I used a lens cloth and dabbed at the contacts rather than blow them so as not to blow the erasings inside the camera.

Try the following diagnosis steps: -

To isolate the cause of the issue, do the following: -
1. Turn off the camera.
2. Remove the lens, battery, and CF card.
3. Allow the camera to sit without power for approximately 20 minutes.
4. Insert only a fully charged battery, and turn on the camera.
5. Depress the shutter button as you would to take a picture.

Does the “ERR 99" message appear? If so, the camera may be faulty and requires a service. If it does not, then: -
1. Turn off the camera.
2. Insert the CF card.
3. Turn on the camera.
4. Format the CF card in the camera.
5. Depress the shutter button as you would to take a picture.

Does the “ERR 99" message appear? If so, then the CF card is faulty. Try a different card. If the message does not appear: -
1. Turn off the camera.
2. Clean the gold contacts on the lens by GENTLY rubbing them with a pencil eraser or soft cloth. Keeping the camera face pointing down, also clean the gold contacts on the camera, being careful that you do not let any debris fall into the camera body.
3. Reattach your lens.
4. Turn on the camera.
5. Depress the shutter button as you would to take a picture.

If the “ERR 99" message only appears when one particular lens is attached, then that lens should be examined by a service technician.
If you see the “ERR 99" with a different Canon lens attached, then the camera is faulty and should be serviced.