If using the Make Diagram Layer or Export Diagram Content tool, no diagram with the input network diagram name exists.
If using the Update Diagram or Delete Diagram tool, no diagrams were returned with the parameters currently specified for the tool.
If using the Change Diagram Owner tool, the specified diagram names don't exist.
For the Make Diagram Layer and Export Diagram Content tools, verify that the diagram name set for the Network Diagram Name parameter exists for the input network version.
For the Update Diagram and Delete Diagram tools, verify that diagrams based on the specified diagram template name and with the specified diagram name for the input network version exist.
For the Change Diagram Owner tool, confirm that the diagram names are spelled correctly.