Welcome to iMore! First try a hard reset.
Hold the top or side power/sleep button and the home button at the same time until the Apple logo appears, then release.
If that doesn't work, try signing out of iTunes, then sign back in.
Go to the iTunes Store> scroll down to your Apple ID > Tap ID > sign out. Then sign back in with your Apple ID.
Make sure you're signed in to iCloud, iMessage and Facetime.