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.