UltimateCD diagnostic software

I am trying to make a bootable flash drive with this downloadable diagnostic software: UltimateCD. Even know it says "CD" instructions say that it can be loaded to a flash drive. However, when I install the ISO on the flash drive the, the write is successful but I cannot get my computer to boot from it. I know the flash boot works as that is how I installed Mint 20. Anybody know about this program? The main reason I want the diagnostics is so that I can run stress test on my video card. So if you know of a good video diagnostic program, I would like to here of that as well. Thank you, in advance.