RPI via MAVlink (unable to Arm)

Hi all, I am using RPI 4 and APM 2.8. I am following the link https://ardupilot.org/dev/docs/raspberry-pi-via-mavlink.html to configure my RPI 4 with APM 2.8 via Mavlink. I have managed to get thru Mav> link 1 down. But however, new problem arises - I am unable to:
param show ARMING_CHECK
param set ARMING_CHECK 0
arm throttle

I have tried different baudrate but same results.

This is what I got:

Please help, i don’t understand why. Thank in advance!