The screen write error 20 after taking a picture

Switch the battery out for a different one, or if you don't have a second battery, make sure that one is fully charged. Try taking a picture using a different lens if you have one. If you don't try just taking a picture without a lens to see if you still get the error.

If you are still getting the error, it likely does indicate a mechanical problem inside of the camera. The most likely cause would be the shutter. It is possible to replace the shutter, but it's a very involved repair as it requires disassembly the majority of the camera to get to the shutter assembly. I generally would recommend against trying to repair it yourself unless you have experience working on cameras or a lot of experience repairing delicate electronics. Your better bet would probably be sending it to a third party repair shop to get it repaired (This model is out of service and Canon themselves will no longer repair it).