I transferred some music mp3 files from my windows laptop to the SD card on my moto z play droid. I can find the files on my phone but when I click on the song, I get a message: "couldn't play the track you requested". I would prefer not to use an app, but I will if I have to. I downloaded Google play music, but it isn't working. Is there a different app needed to recognize and play these files? Thanks.
Are you sure they are .mp3 files? If they are .wma files, they aren't supported by the phone (see page 48 of the user manual:
If they are .wma files, you'll need to use a music app that supports those files, such as: