Party to be notified from partner profile must be a user

What causes this issue?

The party to be notified in the partner profile is not of the type
'User'.

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?

Maintain the partner profile accordingly. The identification code for
type 'User' is 'US'.

Error message extract from SAP system. Copyright SAP SE.