Assignment of & cannot be deleted

What causes this issue?

You want to delete an existing assignment to a material group. This is
prohibited as long as value-only materials are still assigned in the
plant master/material group master.

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?

Delete all value-only material assignments from the corresponding
plant/material group assignment(s).

Error message extract from SAP system. Copyright SAP SE.