If you've booted using the UEFI firmware as opposed to using BIOS firmware then your system should make the EFI NVRAM variables available in:
/sys/firmware/efi/vars/
or
/sys/firmware/efi/efivars/
When booting using a BIOS (or the BIOS emulation mode of UEFI firmware) then these variables aren't available.
In fact, as @Santropedro pointed out, the path
/sys/firmware/efi
is missing when booting using a BIOS, which is easier to check.
