I can't update to Mojave.

Explain what happens when you follow the instructions listed in: How to upgrade to macOS Mojave - Apple Support

I would run a firmware update test, i had to do that for HighSierra on my MacBook air back in 2018.

this might sound strange but-

Seems to me there is a lot of OS X install traffic on their end this week, just something i has noticed with other users and message boards of people having trouble upgrading. just keep on trying.

Mojave is one of the most fluid OS X apple has created and worth the extra effort to install.

You should not download Mojave from the App Store, because you can not: if your Mac is compatible for Mojave it is also compatible for Catalina, so that is what you can download from the Appstore. There is another possibility to download Mojave dmg installer via iTunes.