This issue arises when you don't have permission to the necessary objects and fields belonging to the SMS Magic package.
First of all, the professional edition doesn't provide complete access to any object completely and the professional edition also doesn't allow to set/change the "Field accessibility" for any standard/custom object.
So, to enable accessibility to any field, all you need to do is add the field to the "Page layout" of that object and you are all set!
Please ensure that we have granted access to all the fields belonging to the below object:
- SMS History
- SMS SenderID
- SMS templateType
- SMS Templates
- The object to which you are sending the SMS to