If you are using the Copy Monitor tool with your Levels, and you need to rename the Levels in your Host file. This is usually to keep a consistent naming convention across the entire project.
As soon as you try and rename you host level you will get this error message dialogue box.
Revit doesn’t realise that you are just trying to update your Level name in the host file to match the linked file and now adds a warning to the list. So you don’t get a warning just stop the monitoring of your Level line prior to renaming and then re-monitor the levels after the renaming process.