UoM group & assigned to a material; not possible to change or delete

What causes this issue?

The unit of measure group has already been assigned to a material and
therefore cannot be modified or deleted.

How to fix this error?

Once stock is posted to a material, you cannot change the unit of
measure group for the material. The unit of measure group has to be
identical for all plants of a given material. For additional
information, read SAP note 145824.
It is recommended to use the UOM group enhancement report
?R_OIB_01_UOM_ADD_INSERT_SUM? to add a new UOM. Please, refer to OSS
Note 2328459 for usage & implementation
If any other methods are used, it can create severe inconsistency in the
database tables.

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.