What worked for me to fix this:
I deleted the folder C:\Users\YourName\AppData\Local\UnrealEditorFortnite
(don’t worry, it will regenerate automatically).
Then, I created a blank map and launched a session it worked then After that I opened my main map and launched a session it worked/
Disclaimer: I’m not sure why this happened or exactly what I did, but this method worked for me and is safe, Also you need to start a session in a new blank map first then your main map.