Song cannot be played on this iPhone

iTunes might be refusing to sync the tracks because they use an encoding format that it knows isn't supported on iOS devices. As an example, the iPhone 5s technical specification on Apple's website lists the supported bitrates for constant rate MP3s as "8 to 320 Kbps". If the bitrate for the files is outwith those limits that might be the cause of the failure to sync the files.

You could try using the "Create AAC Version" option on the right-click context menu in iTunes to create a copy in AAC format of one of the affected tracks, then attempt to sync the AAC copy, to see whether it is the format of the file that it is objecting to.

You can also confirm the bitrate of the files by right-clicking on them and choosing Get Info.

I had this issue this morning. I had to force quit iTunes, unplug my iPhone, reboot it, then reconnect it and sync the playlists that wouldn't sync earlier. It seemed to work, as I now have those playlists. I think iTunes experienced a technical issue somehow with its encoding process.

One way to confirm whether the files are really just MP3 files is to open them in QuickTime and then open the Inspector (hold ⌘ and tap I or choose from Show Inspector from Window menu). The Inspector will tell you the file type (if it recognises it).

If these are MP3 files it should say something that starts with "MPEG Layer 3" for the format. If it doesn't then either it isn't an MP3 file, or it is but one that is corrupted in some way.

Assuming these are simple MP3 files, having looked at other reports of people getting the same error message, have you tried removing the files from your iPhone and syncing them back over again. It seems people can get this error if the files are corrupted while syncing.