What causes this issue?

This object type cannot be changed to an associated category.

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?

The object type contains the associated object types that are maintained
in the sub view underneath.
Please delete the dependent records from the sub view and change the
object category to "associated type".

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.