Error 20 is an indication of a mechanical failure (generally the mirror or shutter mechanism). It can indicate either an actual component failure, which would require service or a momentary fault that can in many cases be reset.
Try a "hard reset". Remove the lens, memory card, main battery and the "date battery" using the instructions below. Switch the camera ON an allow it to sit idle minutes. switch the camera OFF, reinstall the two batteries and the memory card, and set the time and date. Check the display, put the camera in M (manual) and fire the shutter. If it operates correctly (shutter fires), remount the lens and try the camera in auto mode and let me know the result.
The date battery holder is at the bottom right of the camera. open the cover that covers the usb connection. At the bottom there is another cover that requires a small screwdriver to remove. That is where you will find the Date battery.
Please keep in mind that my diagnosis & solutions provided are directly dependent on the accuracy of your description of the problem. As with any "do it yourself" fixes, success is a "team effort", since I can't see or touch the camera, and relies on the customer's manual dexterity and ability to follow the instructions well.
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