When I launch UEFN it gives me an error code

@PWR_Juki Thank you for your report! We would like to look into this further, would you be able to submit a bug report using the form available here? Fortnite Creative

Hi Juki, Welcome to the Forums.

The first thing to do would be Verifying your Fortnite files.

Click on the 3 dots next to Fortnite in your launcher → Manage → Verify files

Let us know if you’re still unable to launch.

I checked the files and it still doesn’t work.

Thanks for doing that.

I did a google search for “uefn corrupt pak index” and found a forum post suggesting to reboot your PC and/or Uninstall and Reinstall completely.

But before uninstalling/reinstalling, try deleting the mentioned file from ‘FortniteGame\Content\Paks’ + launching. (If still no luck, try running ‘verify’ again after deleting it) (Info from this Reddit thread)