[Sony Xperia XZ Premium Guides, News, & Discussion] zniesmaczony:
Here you go

16-3-2018_00-34-28 ===> Flash TWRP

target reported max download size of 536870912 bytes
sending 'recovery' (21692 KB)...
OKAY [ 0.465s]
writing 'recovery'...
OKAY [ 0.003s]
finished. total time: 0.468s
16-3-2018_00-34-34 ===> Boot TWRP

downloading 'boot.img'...
OKAY [ 0.550s]
booting...
OKAY [ 5.788s]
finished. total time: 6.338s
16-3-2018_00-38-55 ===> Mount /system

error: no devices/emulators found
16-3-2018_00-38-58 ===> List /system

error: no devices/emulators found
16-3-2018_00-39-01 ===> List credmgr files

error: no devices/emulators found
16-3-2018_00-39-02 ===> Remove credmgr files

error: no devices/emulators found
16-3-2018_00-39-03 ===> List credmgr files

error: no devices/emulators found
16-3-2018_00-39-04 ===> Pull system file

adb: error: failed to get feature set: no devices/emulators found
16-3-2018_00-39-05 ===> Push system file

adb: error: failed to get feature set: no devices/emulators found
16-3-2018_00-39-06 ===> Push lib file

adb: error: failed to get feature set: no devices/emulators found
16-3-2018_00-39-07 ===> Push lib file

adb: error: failed to get feature set: no devices/emulators found
16-3-2018_00-39-08 ===> Push lib file

adb: error: failed to get feature set: no devices/emulators found
16-3-2018_00-39-09 ===> Push lib file

adb: error: failed to get feature set: no devices/emulators found
16-3-2018_00-39-10 ===> Change file attributes

error: no devices/emulators found
16-3-2018_00-39-11 ===> Change file attributes

error: no devices/emulators found
16-3-2018_00-39-12 ===> Change file attributes

error: no devices/emulators found
16-3-2018_00-39-13 ===> Change file attributes

error: no devices/emulators found
16-3-2018_00-39-14 ===> Change file attributes

error: no devices/emulators found
16-3-2018_00-39-15 ===> Change file attributes

error: no devices/emulators found
16-3-2018_00-39-16 ===> Change file attributes

error: no devices/emulators found
16-3-2018_00-39-17 ===> Change file attributes

error: no devices/emulators found
16-3-2018_00-39-18 ===> Change file attributes

error: no devices/emulators found
16-3-2018_00-39-19 ===> Change file attributes

error: no devices/emulators found
16-3-2018_00-39-20 ===> Change file attributes

error: no devices/emulators found
16-3-2018_00-39-21 ===> Change file attributes

error: no devices/emulators found
16-3-2018_00-39-22 ===> Change file attributes

error: no devices/emulators found
16-3-2018_00-39-23 ===> Change file attributes

error: no devices/emulators found
16-3-2018_00-39-25 ===> Change file attributes

error: no devices/emulators found
16-3-2018_00-39-26 ===> Push XperiFIX Flasher

adb: error: failed to get feature set: no devices/emulators found
16-3-2018_00-39-27 ===> Push Magisk

adb: error: failed to get feature set: no devices/emulators found
16-3-2018_00-39-28 ===> Run Xperifix Flasher

error: no devices/emulators found
16-3-2018_00-39-29 ===> Flash Magisk

error: no devices/emulators found
16-3-2018_00-39-30 ===> Wipe Cache

error: no devices/emulators found
16-3-2018_00-39-31 ===> Wipe Dalvik

error: no devices/emulators found
16-3-2018_00-39-32 ===> Reboot

error: no devices/emulators found

EDIT: i swiped right when asked if you want to allow system modifications. I also made sure that the "mount system partition read - only" box is unchecked. Perhaps I should check the System box to be mounted in the first place?