No selected set is assigned to the class characteristic

What causes this issue?

To link a master inspection characteristic with a class characteristic
of type "CHAR", a selected set must be entered in the class
characteristic.
This is not the case for the class characteristic you have entered.

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?

Select another class characteristic.

Error message extract from SAP system. Copyright SAP SE.