Confirm consent before you can send a campaign 
Before you can send your first SMS campaign, you need to get consent from your customers.
Launch the SMS consent dialog by clicking Marketing on the navigation bar at the top of your account. This will take you to the Campaigns page for the marketing center.
On the Campaigns page, click the rounded SMS CONSENT button, which is located to the left of the NEW CAMPAIGN button.
This will launch the consent modal, allowing you to see which customers you've already gained consent from, you can also select new customers from the list by clicking the box next to their name(s).
Make sure you check all the pages of customers by clicking the arrow button located at the bottom right of the consent modal.
To learn more about SMS marketing consent view the consent guide here.
Once you have selected the customers who gave you SMS consent, select the rounded blue Next button. On the next page, click the checkbox stating you have explicit consent from your select customers to send them SMS marketing, then click CONFIRM SMS CONSENT in the lower right.
SMS CONSENT ERROR:
You may get an error saying certain customers could not get consent confirmed. This usually happens if the customer does not have a phone number listed on their profile or you already have that same mobile number under another customer.
Any customers that do not show up in the error dialog were successfully verified. You can press the Cancel button to exit the Modal.