Different plants not permitted within an equipment hierarchy

What causes this issue?

The <GL:superior_equipment>superior piece of equipment</> and the
the sub-piece of equipment that you are trying to allocate to it
have different <GL:maintenance_plant>maintenance plants</> in their
master records.

System Response

INCLUDE 'KEINE-VERARBEITUNG' OBJECT DOKU ID TX LANGUAGE E

How to fix this error?

You can only allocate pieces of equipment to one another if they have
the same maintenance plant. Enter a different piece of equipment or
change the maintenance plant before you make the allocation.

Error message extract from SAP system. Copyright SAP SE.