Hi.Welcome to justanswer.
Can you check the format of the songs (i.e. .mp3, .wma, .mp4)?
Okay no problem. Does this message comes for all the songs or some specific ones?
On your Windows system, right click any of the song and go to Properties. Tell me what is the format written there.
Do you have SongBird installed on your computer?
Uninstall it and then again download it. And tell me how you are adding songs to your mp4 player? By song bird or by some other method?
If you want I can remotely connect to your system an troubleshoot for you. Do you want to start a Secure Remote Assistance session?
CLICK HERE and press I Agree. A file will get downloaded. You have to run that file.