Sorry to necro this, but I came across the same issue and got to this post while searching for answers. @SergeyRuss back in 2019 was on to something and led me to find something that hasn’t been mentioned that worked for me. - Instead of duplicating the level and such, just close the project completely and then open up your project’s “Content” folder in your OS’s file browser. Navigate to where you have the level asset saved and check to see if there is already a BuiltData asset there for the map. If so, delete it while the project is closed (only that file, not the map too). If you do it while the project is open it won’t let you delete it and is the same reason the editor fails when trying to save over it, it can’t. Once the asset is deleted reopen the project and build the map again. It should then let you save the BuiltData asset.
Again, sorry to necro this. Just hope this helps someone else that might stumble on this some day.