Boots up ok from USB flash drive, but it can't find my Hard Drive to install it

I’m trying to install FYDE os intel Legacy on my PC which is running Chrome os flex. I can boot up using the USB boot, but when I try to load onto my PC drive it can’t find my drive. Computer is Lenovo, intel i5. Does anybody have any idea why? Also note, I have an old external hard drive that I hooked up to the computer, and the USB boot drive did find that drive.

Fydeos need UEFI boot.

Thank you “hacker”.
My reseach shows you’re right, but my problem seems to be I can’t find a way to access UEFI boot up. My bios tabs for security and startup give me no way to change to UEFI. I suppose there is probably a way to do it by using terminal mode and putting in some codes. But thats beyound my knowledge base. If you don’t have any further suggestions for me, I’ll continue to search the web for answers. But once again I appreciate your input. Richard