Split Fusion Drive (again)

The Mountain Lion Recovery can not format a drive for a later macOS installation. You maybe able to reformat and install Mountain Lion, then upgrade the later macOS, but that's the long way around.

What you need to do, is startup from a later macOS Recovery partition or a later macOS version on another drive.

If you can not get into a later Recovery partition or have another working Mojave/Catalina drive to startup from?

Then your best bet, is to make a Mojave or Catalina installer on a USB Flash Drive to startup from. Then use it to re-format the drive and reinstall the macOS. see > Create a bootable installer for macOS - Apple Support

 Start up from macOS Recovery

To start up from macOS utilitRecovery, turn on your Mac and immediately press and hold one of the following combinations on your keyboard. Command-R is generally recommended, especially if you never installed macOS Sierra 10.12.4 or later… How to reinstall macOS - Apple Support

Command (⌘)-R

Install the latest macOS that was installed on your Mac.

Option-⌘-R

Upgrade to the latest macOS compatible with your Mac.

Shift-Option-⌘-R

Install the macOS that came with your Mac, or the closest version still available.

If your Mac has a Fusion Drive and you see a flashing question mark or alert, see the troubleshooting section of the Apple Support article About Fusion Drive, a storage option for some Mac computers.

If you continue to have problems with your disk or it can’t be repaired, it may be physically damaged and need to be replaced. For information about servicing your Mac, see Find out how to service or repair your Mac.

In Terminal…

diskutil resetFusion

How to fix a split Fusion Drive - Apple Support