The input Reviewer Workspace contains no Reviewer Sessions. The input workspace may not be a valid Reviewer Workspace. The connecting user may not have permission to access the Reviewer tables.
Verify that the Reviewer Workspace contains the required Reviewer tables and that the Reviewer Workspace contains Reviewer Sessions. If connecting to an enterprise geodatabase, verify that the user specified in the connection file has permission to access the Reviewer tables. You can create Reviewer Sessions using the Session Manager or the Create Reviewer Session tool.