The banner appears at the top of every SMS page while your US/Canadian account has not yet finished registering under the A2P 10DLC rules. It explains the three requirements — identify your brand to the carriers (brand registration), register the type of messages you send (campaign registration), and associate your phone numbers with that campaign — and warns that no SMS will be sent until you have a valid campaign with at least one number. Click Start from here! in the banner to go to the campaign registration page. The banner disappears once a verified campaign with an attached number is in place.