Update: 7/24/2021 4:50PM
It looks like NVIDIA has resolved the issue, as the Fortnite login issue is no longer listed on the GeForce Now Status Page. You should be good to game!
Fortnite is experiencing some login issues
Identified - We are seeing an intermittent issue of users not being able to login into Fortnite for connected accounts. We are actively working to mitigate this issue.
Thank you for your patience!
Jul 23, 17:24 PDT
We will update this article with any updates from NVIDIA.