hi @NineAce
This is not the UEFN its the Epic Games Launcher. (EPGL)
Uninstall The current EPGL using Windows Uninstaller.
Restart PC
Download a new version of the EPGL via search engine to find this
Select Auto Update in EPGL
Now login to EPGL using your email address/password
Make sure you can see and launch Fortnite.
Launch Fortnite
Close Fortnite
Check that Fortnite is ready to launch
In EPGL launch UEFN
This should login
if not Restart PC
and follow