need to boot T2 machine from external mojave disk

I understand I can't boot the main drive from an earlier OS. I thought if you adjusted the startup security settings you could boot from an external drive instead of setting up a VM. I adjusted the startup security to allow boot from external drive and to allow any version of OS signed by Apple. I tried to reinstall Mojave on the external drive from the new machine because I undersatnd if puts some personalization thing on but it wouldnt even let me open the installer. This is actually annoying.

To clarify, the external drive is a bootable clone of my mojave drive so it already has mojave installed but the error message said the system needed to be reinstalled on the external drive. Now I understand this is the standard error message, not an actual response to a problem.