Hello pyaden12,
Thank you for reaching out in Apple Support Communities. I understand an MP3 player that is connected to your Mac is not being recognized. I'll be happy to help.
I recommend to connect the MP3 player in more than one USB or USB-C port and see if that helps. This will help determine if there's a specific port that is not working. Also, while the MP3 player is connected, I suggest going to the menu bar, select the Apple icon > About This Mac. Select the "Overview" tab, then click System Report. You'll see a window appear. Check and see if the MP3 player is listed below Hardware in the list on the left.
Now if these steps do not help, I encourage for you to contact the manufacturer of the MP3 player. You'll want to see if there's an available software update or driver for your MP3 player, that needs to be installed on your Mac.
Kind regards.