Partner type of the partner function entered is not allowed

What causes this issue?

You entered a <GL:partner_function>partner function</> that refers to a
partner type that is not allowed here. You can only
enter partner functions with type "KU" in the customer partner function,
and partner functions of type "AP" for the contact person. The following
partner types are allowed for internal partners:

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

"US" = user
"PE" = person
"O " = organizational unit
"S " = position
"LI" = vendor

System Response

How to fix this error?

Enter an allowed partner type. Check the partner determination procedure
assigned to the notification type.

Error message extract from SAP system. Copyright SAP SE.