Required entry (customer number or customer tax group) not made

What causes this issue?

You have not entered the customer number or the customer group for
which the license is valid.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Specify the customer or customer tax group for which the license
applies.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.