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)