Customizing of object category &1 is incomplete

What causes this issue?

System Response

You cannot assign objects with this category in the notification
application. You also cannot search for notifications using an object
with this category.

How to fix this error?

Inform your system administrator. They can check whether an active BAdI
implementation exists for the object category and can complete
Customizing for the object category.

Procedure for System Administrators

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.