Everytime I put a CD in my factory installed CD player (1998 Sunfire) I get "error 20" and it spits my CD back to me. Radio works fine. Please help me to understand what error 20 is and how it can be fixed.

you can either

A- have a friend pop your cd player out for you and see what the problem is

B- take your sunfire to a certified GM dealer and have them fix it (probably won't be cheap but not a crazy amount either)

C- take it to a shop that only does audio and detailing and what not

D- get one of those cd drive cleaners from your local staples or office depot, the ones for slot drives, and see if it was just a dirty lens.