There must be an attribute for the RDN mapping

What causes this issue?

You have assigned the RDN flag to one or more mappings, meaning that
the number of attributes is not equal to 1.

System Response

The mapping cannot be saved or used.

How to fix this error?

Select a mapping for the RDN that contains only one attribute in the
directory.

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.