I had the same problem. I fixed it with this:
- Make sure you close uefn and fortnite before. Open the folder of the project you’re having the issue with in windows (in uefn click project>show project in finder to find it).
- find the .urc folder and drag it out to your desktop. (don’t delete it just yet incase)
- Reopen uefn and open your project, it should make a new .urc folder and reset state. Then you won’t get that error again.
I found this solution from a similar error with control revision, but it worked for me on this specific issue too.