Did the T2 Chip got an upgrade?
The solution requires a Mojave Installer USB disk. First download the latest Mojave installer. You can do so from by following
this App Store link.
If you are running Catalina, you can also use the new option in softwareupdate:
> softwareupdate --fetch-full-installer --full-installer-version 10.14.6
Then you can use the createinstallmedia command in the Install macOS application to build an external installer Drive on a USB drive. You probably want to add the option to add the current firmware to the USB drive as well.
> createinstallmedia --volume /Volumes/Untitled --downloadassets
This will delete the target volume data on the USB disk.
When you run the Mojave createinstallmedia tool without arguments you get the usage documentation:
$ /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia
Usage: createinstallmedia --volume <path to volume to convert>
Arguments
--volume, A path to a volume that can be unmounted and erased to create the install media.
--nointeraction, Erase the disk pointed to by volume without prompting for confirmation.
--downloadassets, Download on-demand assets that may be required for installation.
Example: createinstallmedia --volume /Volumes/Untitled
This tool must be run as root.