Mojave OS

One of my iMacs is still running Catalina. I created a new APFS volume, created a Mojave USB bootable installer, booted the iMac with it and installed Mojave to the new volume. Can boot to either Catalina or Mojave.

How to create a bootable installer for macOS - Apple Support

You need to add some details as to what you are trying to do. Mojave will certainly run in an APFS volume.

I am running Mojave without issues on a 2012 MacBook Pro with an aftermarket SSD formatted APFS.