In this case, most common cause of the problem is that the purchased license includes smaller range of devices than there're currently assigned into the system, e.g. 10 endpoints and 1 server devices, and the client has assigned 3 server appliances on trial license. To resolve this problem you have to delete assigned excess devices by Management Center.
The other case can be that during purchase the customer has provided incorrect S/N.
If the problem with upgrading the license could not be resolved, please contact our technical support.