Operating System Not Found after Install over Win 10

I'm trying to install Mint Linux Cinnamon on an old Dell Laptop. On restart after what looks like a good install from USB, I get an "Operation System Not Found" error. Boot order in BIOS is correct (HDD at the top). Online troubleshooting suggests I need to be in UEFI boot mode. No such option exists in the laptop BIOS. The laptop was sold with Win7 and eventually "upgraded" to Win10. Apparently UEFI post-dates Win7. I'd like to find a solution. If this laptop is likely incompatible, it'd be helpful to know that.