To elaborate a bit on Fabby's answer:
Modern EFI/UEFI-based computers can usually boot either in native EFI/UEFI mode or using a feature called the Compatibility Support Module (CSM), which enables them to boot using older BIOS-mode boot loaders. This latter mode is often called "legacy mode," so it has three names: BIOS, CSM, and legacy; all mean the same thing in this context, and are distinct from native EFI/UEFI-mode booting.
Secure Boot is a UEFI feature that is active only in UEFI-mode booting. Thus, when you activate BIOS/CSM/legacy support, Secure Boot is either automatically deactivated or becomes relevant only for UEFI-mode boots, depending on the design of the firmware.
If your computer is booting in BIOS/CSM/legacy mode and refuses to boot when you switch the firmware to EFI/UEFI mode, then that implies that Secure Boot is already disabled, or at least bypassed, for booting Windows. Fabby asked for the output of
parted -l, and that output will indicate Windows' boot mode with certainty. Specifically, Windows boots in BIOS/CSM/legacy mode only from MBR-partitioned (aka "MSDOS") disks, whereas it boots in EFI/UEFI mode only from GPT-partitioned disks. Thus, the disk type will indicate, with 100% certainty, what the current Windows boot mode is.
As a side note, any instructions that tell you to disable Secure Boot as a matter of course are suspect. Ubuntu has long supported Secure Boot, and should install just fine (in EFI/UEFI mode, of course) on most computers with Secure Boot active. Note the word "most," however; there are a few computers on which Ubuntu's Secure Boot support does not work. IMHO, those are best dealt with on a case-by-case basis rather than issuing instructions to disable Secure Boot on all computers.