The first thing to check is that the Primary contact does have a mobile number on their contact details. As a lot of the time, the mobile number will be under Phone (especially if they only have a mobile).

You may need to Edit the contact to make sure that the correct information is there:

Remember to "Update Contact" if you do need to add their mobile number!

On the Client Overview you will see under the "Create New" an option for SMS

These are found under Configuration > System Settings > SMS Templates

If you need to create one, you will need to go to the green "Create New"button

The Custom Tags are to fill in relevant information with regards to client name, renewal date, etc.

Go to the Client Overview, select "Create New - SMS" and add the mobile number and the template.

This is what will come up and then press the green "Send" button.