Cannot remove material &1; it is not yet assigned

What causes this issue?

Material &v1& cannot be removed as it is not yet assigned.

System Response

The material to be removed from the material assignment table is not
assigned yet and cannot be removed. Changes on the material assignment
could not be performed.

How to fix this error?

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.