how do I fix my mac OS Mojave installer?

You have to have a 2012 MBA or newer in order to run Mojave. If it can run Mojave it can also run Catalina.

Look in the Applications folder for a file named Install macOS Mojave.app. If it's there delete it and try downloading again.

The App Store will not permit you to download Mojave unless it is supported on your specific model. If your model can run Mojave, then it can also run Catalina. The App Store will let you download Catalina but not Mojave in this case.

To find the complete model information select About This Mac from the Apple menu. You can create a snapshot of the dialog as follows:

How to take a screenshot on your Mac

Capture, Save or Record Screenshots in Mac OS X - Help Desk Geek

OS X Screen Capture Shortcuts

(⌘⬆︎3) Command-Shift-3 Capture the screen to a file

(⌘⬆︎⌃3) Command-Shift-Control-3 Capture the screen to the Clipboard

(⌘⬆︎4) Command-Shift-4 Capture a selection to a file

(⌘⬆︎⌃4) Command-Shift-Control-4 Capture a selection to the Clipboard

(⌘⬆︎5) Command-Shift-5 Invoke new screen grabber

You can then post the image in the forum's message editor.