Close background apps by double tapping home key, press and hold any icon that appears in bottom task bar, press red x that appears on each icon when they start to wobble. This closes, does not delete app. Close all of them.
Then restart iPad. Press and hold power button until power off slider appears then slide it to power off the iPad. It may take a moment or two to power down if memory is taxed.
Restart iPad.
Attempt to access iTunes.
If problem persists go to settings, general, restrictions, and make sure no restrictions are active that could prohibit iTunes access.
If restrictions are disabled then try settings, store, and sign out of the store then sign back into the store, and view account to assure there isn't a problem with the account.
If all of these fail I would suggest signing into store with another ID to see if problem persists.
Good luck