Key field attribute set to 'Get Value at Activation'

What causes this issue?

You have selected a key field which is <ZH>not</> to be put in the BC
Set.

System Response

All key fields of a table are required, to assign data records uniquely
in a BC Set. All key fields which are <ZH>not</> to be put in a BC Set
are given the attribute ''<ZK>Variable</> (Get Value at Activation)'

How to fix this error?

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

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.