Failed to mount system_ext (block device required)

[mrstarnge]

I rooted my op7t and i am on official oxygen os 10 with latest twrp 3.7.0 and magisk installed. It is having this error i don't know why. Someone kindly explain and help.

I flashed twrp using fastboot flash recovery recovery.img and tried flashing in both boot as well using :

fastboot flash recovery_a recovery.img
fastboot flash recovery_b recovery.img