can't install High Sierra on volume created in Catalina- disk is locked

I have Catalina installed on my Mac mini. I created a volume to install High Sierra to continue to use software that I need. I have tried multiple ways to install on the new volume, including creating a High Sierra boot USB. No matter what I try, when I get to the screen to select the volume to install the OS on, all of the volumes say "this disk is locked." There is no firmware password on the drive. FileVault is not turned on. I've wiped the drive three times. How and why is the disk locked?

I know it's possible to do this because I did the exact thing on my MacBook Pro last week. I upgraded it to Catalina and then created a volume and installed High Sierra on it. Everything is running great.

Thanks in advance for any light you can shed on this mystery.

Mac mini 2014

3.0 GHz

2 TB Fusion Drive

MacOS Catalina 10.15.1