Can't upgrade to Mojave

Did you read Section 1 of the compatibility table to see if you mac can install Mojave.

As I said and Lexiepix if it cannot install Catalina it cannot install Mojave.

As Lexiepix said try High Sierra,

How to upgrade to macOS High Sierra - Apple Support

Make sure your mac is connected to your router using a cabled connection.

and read the instructions.....

If your mac cannot upgrade to Catalina it probably can't upgrade to Mojave.

Look at this link, Section 1

and the link does work, use Safari as your browser there are problems with Firefox.

How to upgrade to macOS Mojave - Apple Support

Yes, you need another link to download Sierra or High-Sierra, not in the Appstore (but in iTunes):

Download macOS Sierra

or

macOS High Sierra laden

Lex

If too old for Catalina, you can not install Mojave either.

Try HighSierra.

Hmmmm, sync in the communities is rather slow again here...