I got it working. It turns out the LSI adapter can have varying levels of being disabled. And there appears to be a bug. I had it entirely disabled, yet it let some OSes could still see the disks behind it. Windows in UEFI mode could install to them, but the system can’t boot from them at all. Windows in BIOS mode could not install to them, correctly noting the adapter was BIOS-disabled. Once I enabled it, I can now boot from Linux.  By the way to enable the card, one must boot from something; otherwise, the adapter control utility won’t load. Thanks to all who answered.
I’m resurrecting an R820 with internal disks connected to an MPT3SAS-based LSI card. It’s in UEFI mode. I installed Linux to one of the drives. The system can't find any drive to boot from. The system also can’t boot from a Microsoft-created Windows 21H1 USB flash. I managed to install 212H1 via the DVD to an internal drive in UEFI mode, but the system can’t find that either. I will try BIOS booting next, but am I wondering what might be going on to prevent UEFI booting from the internal disks entirely? Could it be it needs a bios/firmware update? The system hasn’t been updated in several years.
R820 doesn't want to boot from the internal drives in UEFI