Plan characteristic &1 is already being used as an attribute

What causes this issue?

You want to include characteristic &V1& as a plan characteristic. This
characteristic is already used as a master data attribute for the plan
characteristic &V2&. This is not possible for technical reasons.

System Response

You cannot use the plan characteristic.

How to fix this error?

If you want to use the plan characteristic, first delete the attribute
assignment. You can then include the plan characteristic.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.