Download Termux terminal emulator
TWRP image file
Place the multi-disabler.zip on sdcard
Open magisk manager - install - select and patch an image , select the TWRP image
Reboot [don't forget to reboot to magisk with button combo - bixby, power & vol up]
Open Termux and enter this:
Su
f=/storage/emulated/0/Download/magisk_patched.img; dd if=$f of=/dev/block/sda15 bs=$(stat -c%s $f)
Reboot to TWRP
power, bixby, vol up , let go of power and keep holding bixby and volume up
In TWRP flash the multi-disabler zip - format data type YES
Reboot into magisk.
Note: it may take several reboots for magisk to execute the root.