1. Have you ticked the SMS permission box - On a contact there is a section for communication preferences/permissions, with everything from Email to SMS there. It is important that you have permission to communication with contacts via the different methods, if you do have their permission (opt in), you need to make sure the appropriate box is ticked as that will tell the system you have permission to communicate with them in that way.
2. Have you checked they haven't opted out/unsubscribed - With all communications the contact can opt out, with an SMS this would be the contact replying with a simple message of "STOP" which the system will then automatically stop you from sending future campaigns.
3. Have you checked that the number is correct - Depending what country you are in will determine what format, length, etc the phone number needs to be in
4. Have you got enough credits to send an SMS - with emails, you get a monthly allowance but SMSs are purchased separately, if the system isn't allowing you to send an SMS it could be that you need to purchase credits
5. Have you got the correct account to send an SMS - with the different packages of Sales, Marketing and Complete, only Marketing and Complete account types have all of the communication methods, such as SMS available to them

If you are still having trouble sending an SMS campaign, please raise a support ticket