QUESTION AND ANSWER
Why can't I assign a Service to my Customer?
GoToAssist Service Desk
If a Service cannot be added to a Customer, most likely the Service is not added yet to the Company the Customer has been assigned to.
Check to see if the Customer is assigned to a Company. If the Customer is assigned to a Company, then check to see if the Company has the Service added to it, if not, add the Service. Once the Service has been added to the Company, it can then be added to any Customers within that Company.