I also had this issue today and found a work around just now. For me, the new layout didn't show a title on the tab for the new layout. I'm not sure if this has anything to do with it or not, but I renamed the problem layout from the Catalog pane and then saved the project with the problem layout. I then saved and closed ArcPro. I re-opened the project and when I added a new map frame and like magic it worked. Before this solution I had tried 1. deleting the problem layout and just inserting it again and got the same error; 2. deleting the problem layout and restarting arc pro and got the same error after inserting a new layout; and 3. re-starting my system, starting arcpro, inserting a new layout and got the same error after inserting a new layout. To see if the issue was permanently fixed in that project I tried adding another layout. This time the layout tab was appropriately labeled, and a map frame loaded just fine. However, on a whim, I restarted arcpro and added another layer and the layout tab was once again not labeled and did not want to load the map frame. My conclusion, if you add a new layout and the tab is blank try the above work around; if it is appropriately labeled with "LayoutX" you can probably proceed as usual.
... View more