iTunes file type not supported on iPhone

When I sync my iPhone 6 with my computer, I get an error message saying that the file types of the music I'm trying to add are not supported by the iPhone. The weird thing is, is this is happening with files I bought form iTunes and have had on my phone before. It removed almost all my songs from my phone and I can't add them back because I get stuck with this error message. Has anyone else encountered this or know how to fix it?