Error code 20 appears when I turn on rebel t3i

@cuitm this is a general error code which indicates a mechanical malfunction in the camera, so not very specific.  Luckily this camera only has a few mechanical components so you do want to check your reflex mirror and the focal shutter. Most commonly this is actually caused by a mechanical issue with the lens. Try another lens and see if that resolves the issue. If you do not have access to another lens, remove the current one and take a picture without the lens (you will of course not get a picture;-). If you can verify that the error code no longer shows, you know that you have a bad lens.