Have you looked to see what the minimum OS requirements are for running Fortnite,
in all likelihood it will probably need a mac running a more modern OS.
Click on this link,
https://www.epicgames.com/help/th/fortnite-c75/technical-support-c118/what-are-the-system-requirements-for-fortnite-on-pc-and-mac-a3332
As you can see you need a mac running a minimum of macOS Mojave 10.14.6,
macs from 2012 and newer can run Mojave, so can you upgrade your mac.
Click the top left of your screen and click on About This Mac.
Open Safari and click on this link,
How to get old versions of macOS – Apple Support
Go to Check Compatibility to see if your mac qualifies for macOS Mojave.
Then go to the Download OS section and click on macOS Mojave.
Before upgrading make a full back up of your Mac.
Check that third party apps you use have updates available for the new OS.
https://roaringapps.com
Check that your printers/ scanners etc have updated drivers and software available for the new OS.
Connect your mac to your router via cable rather than rely on WiFi.
Do not use your mac while it is downloading
and do not use any other devices that share the same internet connection.
If the App Store says ‘Open’ then it is detecting a previous download of the install app
go to the Applications folder locate Install macOS Mojave.app.
Move it to the Trash, empty the Trash.
Restart, click on the link again, the App Store should say ‘Get' or 'Download'.