Examine the disk to see if it is already burned.
Try other writable CD's.
Try burning a different playlist.
What files are you trying to burn? Apple music is a streaming service & those files cannot be burned to CD please examine the playlist if you use that service. Remove any Apple music files from the playlist & retry burning.
You can also try another app to burn a CD to check that iTunes is the issue, Burn is a free app or use Finder to burn a disk to confirm the drive is working correctly. Finder will not create an 'audio CD', so don't expect the results to play in a CD player.
http://burn-osx.sourceforge.net/Pages/English/home.html