Your mac can only upgrade to macOS High Sierra, it cannot upgrade to Mojave, Catalina or Big Sur.
Do this.
Open Safari and click on this link,
How to get old versions of macOS – Apple Support
Go to the Download OS section and click on macOS High Sierra.
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 High Sierra.app.
Move it to the Trash, empty the Trash.
Restart, click on the link again, the App Store should say ‘Get' or 'Download'.