Want to boot second O/S from SSD. Not having much success.

I just took delivery of a Delta 3 yesterday. I bought it with the intent to run Linux Mint, so I didn't get an activated Windows. I installed a Samsung EVO 970 1TB SSD in the M.2 "B" key slot.

I was able to boot the Mint install disk from a USB drive. Then I installed Mint on the SSD. But I cannot get the system to boot from that SSD at all. In the BIOS, my UEFI boot order is NVME-USZ-Windows-Network. The LEGACY boot order is: NVME-USB-Hard Disk-Network. But restart does not recognize the O/S stored on the SSD, either with UEFI or Legacy. I tried disabling the EMMC, but then the system would not boot at all (the USB drive had been removed, but the SSD was in).

I'm sure there is an install parameter for Mint that I missed, or that there is a BIOS setting that is wrong, but I am reluctant to use trial-and-error on the BIOS settings, as that "and-error" part may turn the Delta 3 into a brick. Also, although I wanted Linux from the start I am reluctant to overwrite the non-activated Windows as that is the only O/S that starts for the moment, if you don't count USB boots.