There are a few reasons why this can happen:
- Date and time could be wrong on your device. Check that in Settings - General - Date & Time.
- The connection to the internet isn't working. Try to access an random website through Safari. If this is not possible. reset your iPad, also reset your router (turn off and on).
- Go to Settings - iTunes & App Stores, tap your Apple ID and sign out, then sign in again with your Apple ID. Sometimes this resolves issues like yours.