Mojave is still available for download, follow these instructions carefully.
Please use Safari to access the following link do not use Firefox
How to upgrade to macOS Mojave - Apple Support
go to Section 4 and click on Get macOS Mojave
you will be redirected to the App Store Mojave download page.
It should say Get or Download.
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'.