How do I Install Mojave on a Separate Volume of My Mac

Yes, that made the difference. There was something not right with the USB stick holding the Mojave installer. It worked after setting up the installer again on a new external USB unit. Stupid I didn't try that right away, but very much thanks for the info. It saved me additional time. So for all others that get the prohibited sign: Try first to setup the installer on another USB disk, because it has to work.

As you have noticed, you can't open the Mojave installer on your Catalina volume. You can use the Mojave installer to create a bootable Mojave installer on a 16GB USB flash drive. Take a 16 GB flash drive, plug it into your Mac. Open Disk Utility. Click on "View", then select "Show All Devices". Select your 16 GB flash drive media in the Disk Utility sidebar (not the indented volume below it). Click on "Erase". In the dropdown, select Name: MyVolume, Scheme: GUID Partition Map, Format: MacOS Extended (Journaled). Click on "Erase" in the dropdown.

Now copy the command for Mojave from the Apple Support article below

How to create a bootable installer for macOS - Apple Support

Open Terminal and paste the command in the Terminal window. Enter your administrative password. Type Y when prompted. Let Terminal finish. You should now be able to select your newly created bootable Mojave installer as a Startup Disk in System Preferences. Restart you Mac using the Mojave installer as the Startup Disk. You should now be able to install Mojave on the separate volume you mentioned. Just select it as the destination for the install.

I also tried this, but I am getting the prohibited sign when booting from the Mojave installer. I found several places where this approach is mentioned, but none that mention the prohibited sign showing up and what to do about it. I even tried to create a separate partition instead of just a volume, but that didn't work either. Maybe this worked at some point and the latest Catalina update added some extra restrictions? Maybe it has something to do with the Mac model (Mid 2013 iMac in my case)? Maybe the Mojave installer will no longer work when a newer system is detected on one of the volumes? I am a bit perplexed about why I seem to be the only one that can't get this to work.

FWIW, I just installed Mojave on an APFS volume on a Late 2012 iMac running Catalina 10.15.2 two days ago. It may not work if you have FileVault enabled or some other encryption method. I didn't have either.

Thanks. This seems to indicate that something could be foul with my Mojave installer, although no problems are found with Diskutil First Aid. I will try to set up the installer on a different medium.

Glad you got it working.