Material group & is already assigned to material group hierarchy level &

What causes this issue?

Each material group can only be assigned one material group hierarchy
level.

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?

If you wish to change the assignment, first delete the old assignment
and then create a new one or, alternatively, change it using
reclassification functions ("Change").

Error message extract from SAP system. Copyright SAP SE.