Check to see if the device is authorized with the PC> You can verify by going to the Store tab in iTunes, then clicking Authorize This PC and inputting the AppleID used on your device, then try and sync again. If that still doesn't work, click on your device on the left hand side bar in iTunes so that you see the profile page that allows you to backup your device, etc. Scroll down and put a check next to "Manually manage music and videos" and Apply the Settings. Now you can manually drag and drop your music onto your device individually without having to sync them by the conventional way of clicking the Sync button, and, it allows you to select songs individually rather than the entire bulk if applicable