Im able to open the UMAP file
That means there is corruption in the file structure; possibly, the only way to recover this is
open a new project
import/migrate the original project into the new project, basically, a level has broken in the project, and no tools exist to externally fix the project
How to copy a UEFN project - Development / Getting Started & Setup - Epic Developer Community Forums
How to duplicate or copy a UEFN Project Easily! #uefn fortnite
There are loads of videos and articles for copying, but just not any about your error