Only macOS Base system visible after failed Macintosh HD erase. How to reinstall macOS Mojave?

I've got an issue with a Macbook Air running macOS Mojave that wouldn't reboot anymore.

• After restarting in Recovery Mode (Cmd + R) and running the Disk Utility function First Aid, I got:

"problems found with partition map; needs to be repaired"

• I tried to reinstall macOS but got the following message:

Disk Macintosh HD: Can't upgrade this version of macOS because a newer version is installed

Macintosh HD - Data: Not enough free space; additional 6.52 GB is required (11.53GB available)

• Tried erasing Macintosh HD via Disk Utility which resulted in this error:

An Internal Error has occurred; Operation failed

• After a new restart in Recovery Mode Disk Utility shows only

macOS Base System

What steps do I need to follow in order to get a usable partition again to reinstall macOS Mojave to?

All help is greatly appreciated.