You cannot link class characteristic & to the master insp. characteristic

What causes this issue?

To link a master inspection characteristic to a class characteristic,
the class characteristic has to fulfill one of the following criteria:
The class characteristic is of type "NUM".
The class characteristic is of type "CHAR". In this case, a selected
set from catalog 1 must also be assigned to the class characteristic.

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?

Error message extract from SAP system. Copyright SAP SE.