No mapping defined for this filter

What causes this issue?

The "Filter" indicator must be set for exactly one attribute in a
mapping.
This attribute is used when reading objects from the directory to
be able to assign the directory entry to an entry in the SAP system.
For example, in the proposals delivered by SAP for the mapping for
user master synchronization, the attribute "sapUsername" is the filter
attribute, since it is mapped to the databas efield that contains the
SAP user name.
You must set this indicator even when you do not want to import any
data from the directory, since the filter attribute is required to
construct the search request was the existence check for entries
in the directory.

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?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.