ASRock UEFI setup utility has no option to disable secure boot
Alright, I stumbled upon the perfect article on AskUbuntu.com. SecureBoot doesn't show up because my computer doesn't have it. To figure out if you have SecureBoot or not, run Windows PowerShell as Administrator, then type:
Confirm-SecureBootUEFI
It will return true, false, or "Cmdlet not supported on this platform". Mine returned with the latter, and thus doesn't have it.
Link to answer that explained this