I can't repro the problem either. I wonder if it has to do with uninstalling OneDrive and the results of that on the special "Documents" folder. Planner uses the OS to get the path to the current logged in user's 'Documents' folder. This should always work. The warning says that the flat plan is in a different folder from the observing plan or has a different name (not YourPlan-flats). From your screen shot you don't have the "Specify File Name" box checked.

I'm probably going to need to watch you do this so I can see where it's going wrong. What are a few times I could get on live with you? Not today (Sunday).

(moved to the Planner section)