Can't burn newly created playlist to CD with iTunes 12.1

I just want to burn some music for my Mom to a CD-R. I've done this multiple times. If I check MP3 CD box, I receive message stating all music is not MP3, and it won't allow. If I check other option, Data CD or DVD, it copies but if I insert into CD player, message is, " No Audio." This can't be this hard. Any help is appreciated. I have OS 10.7.5 Thanks