Go into the bios setup and look for an option "enable UEFI" or disable legacy mode or something. I don't have an ROG board handy so can't look up the exact name used, but it's just an option that needs to be enabled, that is now being reported as disabled by the diagnostic tool. Look through the settings, ROG bios'es tend to be be huge so take your time, it's there.
Also do not forget if it's in legacy mode now, and windows is already installed, when you try to boot it, it will bluescreen. There are ways to mod it in safe mode so it'll boot again, but I really do not recommend that, unless you really know what you're doing.
Best practice, if it needs to be changed is to re-install your windows completely. In general there is absolutely no reason today to not run in UEFI mode, unless you're dealing with legacy hardware.