As much as i know, openkirin roms dont support TWRP. To install magisk, search for tecalotes guide on how to root.
This is the mantra they repeat over and over on the Telegram channel.
The truth is that TWRP does not support encryption on Oreo, therefore from TWRP you cannot flash (wipe, backup, restore, etc) Data partition (and cannot wipe Dalwik which is also on Data), and cannot read (eg install from) Internal memory.
TWRP can access all other partitions, and Open Kirin operates on System (supported by TWRP) and Data (not supported by TWRP).
Therfore Data partituon must be wiped from stock Recovery.
Combining the two, you can use TWRP, but must be careful
For more info see to the HWOTA7 (rebranding) thread.