Ok let's try this, open Safari, do not use Firefox and click this link.
How to upgrade to macOS Mojave - Apple Support
Read Section 1 and see if your mac qualifies, if so
go to Section 4 and click on Get macOS Mojave.
This will redirect you to the App Store Mojave download page.
It should say Get or Download.
When downloading from Apples servers it is always best to have
your mac connected to your router via cable, if you can only use
WiFi then make sure your mac is as close to the router as possible.
Do not use the mac while it is downloading or any other devices that
share the same internet connection.