Why cant my mp3 player play m4a songs?

Because you have a MP3 player that can only play MP3 audiOS, but M4A can't play on a MP3 player because M4A is an MPEG 4 and your MP3 player is a MPEG 3.

The MPEG 4 is to powerful or high for the MPEG 3 (4 is bigger than a 3).