Using Audacity 3.3 to record

Question

Using Audacity 3.3 to record from old audio tape, All played ok until tried to Export it to a CD or DVD. I then downloaded essential MP3 decoder & ok. BUT when I exported it to either CD or DVD it copied my 21minute file OK in 2 minutes, but said that CD [& DVD]would not accept it. Do I need format CD or DVD before export it in MP3? Or do I need export it in a .WAV etc format?