& cannot be assigned. Material & is assigned to plant &

What causes this issue?

You attempted to assign a material group a new hierarchy level. This is
not allowed if hierarchy materials are assigned to a plant.

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

System Response

How to fix this error?

Delete the material group/value-only material assignment from the
plant/material group assignment.

Error message extract from SAP system. Copyright SAP SE.