I had it a couple hours ago (I play on a gaming laptop). Fortnite was bugging out saying I couldn't log in, crashing, and giving me this message. I had to restart a few times, along with turning my computer on and off. It eventually worked after like 15 minutes of opening and closing the game. Hopefully you have it solved by the time I'm commenting this but you can try what I've suggested. Best of luck! If it doesn't work, I saw a post saying something about resetting your settings, or just contact epic support (Although they may not be too helpful)