Interestingly enough, when I was poking around in the Secure Boot section of the MSI B550-A Pro, I came across evidence that suggests that the signed UEFI Secure Boot keys for Ubuntu & Debian are blacklisted. I just can't get that far with the B550-A Pro and was hoping somebody could help. I took this Ventoy USB stick over to my old system (Gigabyte B85M-DS4H-A / Core i5 4670K / 16GB DDR3) that nothing in it - no HDD, no GPU - and with a little trial and error with Secure Boot settings in the BIOS there, I was able to reach the MOK Manager screen, enroll the key, reboot, and verifed that the Ventoy stick does indeed boot in both UEFI and CSM modes with or without Secure Boot enabled and every ISO loaded in both UEFI and CSM modes, too (except for the Windows ISOs since there's no HDD in the machine).

I never see that screen on this board and poking around in the BIOS for a way to enroll the key is pretty Greek to me.

The issue I'm having is I can't get this USB stick to boot with Secure Boot enabled on the MSI B550-A Pro and according to the Ventoy tutorial here, I should see a blue screen verification error and proceed to enroll the signed key with their MOK Manager. I've created a multiboot USB stick with Ventoy with Secure Boot enabled because my system already has it enabled along with TPM 2.0 and Windows 10 is working fine, however since Windows 11 requires Secure Boot enabled, I want to avoid using either Rufus or the Media Creation Tool and stick with Ventoy as it's already configured and loaded with ISOs of a few Linux LIVE CDs, Hirens Boot CD, Kasperky AV Rescue Disk, Macrium Rescue, Windows 10, and Windows 11. 16GB G.Skill FlareX 3200 DDR4 (dual channel mode)

Ventoy multiboot tutorial