Can you render over a painted wall?

I take it that you mean "paint" over a previously painted wall. If that is the case, yes you can. Your best bet is to clean the wall thoroughly. You can find some special phosphate or lye based cleaner at the hardware store. Be sure to follow the safety precautions like using gloves and eye protection. Next you will need to prime the wall to cover over whatever was there before. Then it will be ready for your new paint.