Perhaps he was trying to install Windows 10 using a USB stick.
The various software for that builds the image for either legacy BIOS or UEFI, and they're not interchangeable. Rufus, for example, lets you specify the mode.
Alternatively, if you can't install it in UEFI mode, you can switch to legacy BIOS/CSM mode in your machine's BIOS (or vice versa).
It gets worse - some Win10 ISOs themselves are made for either UEFI (most common) or legacy BIOS, and they can't be set up the other way.
You can clone or run Win10 legacy BIOS/CSM installations on other machines/drives (useful if you plan on swapping/upgrading), while UEFI will only run on the specific motherboard/CPU you installed it on.
I've also noticed Win10 in UEFI mode can BSOD with no chance of recovery when undervolting too much and causing it to freeze/BSOD. Something must be getting corrupted, and it won't run again, ever. Legacy mode works fine.