Sometimes the setting for it is just not there. It also might be FLOCKDN protection preventing your actions, see Bypass BIOS Flash Protection Range Registers on Insyde BIOS (Xiaomi Air 12.5) - #4 by Wootever
Meaning SMM flashing only is allowed.
I didn’t bother myself figuring out this stuff last year. I liked the way of unlocking without flashing.
If you want to do more serious thing than this, I think it’s time to get an SPI prog.