Customer &1: Mandatory partner role &2 is missing for sales area &3

What causes this issue?

The system checks whether all mandatory partner roles exist for each
sales area.

System Response

The system terminates processing if the check fails.

How to fix this error?

Transfer the mandatory partner roles for each sales area during
creation. You cannot delete mandatory partner roles when making changes.

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.