Usually this happens with an aftermarket battery. Canon puts a chip in their batteries allowing the camera to better monitor the battery's charge, most aftermarket batteries don't have this chip. The "error" saying the camera cannot communicate with the battery usually has a question asking if the user wants to use the battery, just click yes the camera will work normally.

The part number from Canon is CG2-2851-000 for the LCD assembly. It is not very complex, take the cover off the LCD hinge, take off the rear cover, three screws and one plug to remove the LCD.

Generally error 20 is in the shutter assembly, usually it needs to be replaced.