Go to Settings>Messages>Send & Receive at>Tap your Apple ID and sign out. Then go back and turn Messgaes off. Restart your iPad.
Restart the iPad by holding down on the sleep button until the red slider appears and then slide to shut off. To power up hold the sleep button until the Apple logo appears and let go of the button.Go back to Settings>Messages>On>Send & Receive at>Sign in and then see of your correct phone number is available to use in the settings.If it is not available to use - are you using your own Apple ID on the iPad?