Check your date and time settings, it may be set to manual instead of automatic. To get there, look up date and time in the search bar of settings. Next, enable the automatic option. If that does not work, try signing out of iTunes. Do this by going to settings and selecting your profile at the top. Then, select media and purchases which will prompt the option to sign out. Restart your device and after making sure you have a stable internet connection, sign back into iTunes.