[Sony Xperia XZ Premium Guides, News, & Discussion] zniesmaczony:
So, I installed the ADB drivers from clockworkmod.com, launched TWRP after it had been flashed, unplugged the phone after being told to, rebooted TWRP, checked the System box, making sure the read-only box is unchecked, clicked OK and waited until the Fix finishes. No effect.

The log:

16-3-2018_01-07-35 ===> Flash TWRP

target reported max download size of 536870912 bytes

sending 'recovery' (21692 KB)...

OKAY [ 0.468s]

writing 'recovery'...

OKAY [ 0.003s]

finished. total time: 0.471s

16-3-2018_01-07-41 ===> Boot TWRP

downloading 'boot.img'...

OKAY [ 0.548s]

booting...

OKAY [ 5.788s]

finished. total time: 6.336s

16-3-2018_01-11-00 ===> Mount /system

error: no devices/emulators found

16-3-2018_01-11-03 ===> List /system

error: no devices/emulators found

16-3-2018_01-11-06 ===> List credmgr files

error: no devices/emulators found

16-3-2018_01-11-07 ===> Remove credmgr files

error: no devices/emulators found

16-3-2018_01-11-08 ===> List credmgr files

error: no devices/emulators found

16-3-2018_01-11-09 ===> Pull system file

adb: error: failed to get feature set: no devices/emulators found

16-3-2018_01-11-10 ===> Push system file

adb: error: failed to get feature set: no devices/emulators found

16-3-2018_01-11-11 ===> Push lib file

adb: error: failed to get feature set: no devices/emulators found

16-3-2018_01-11-12 ===> Push lib file

adb: error: failed to get feature set: no devices/emulators found

16-3-2018_01-11-13 ===> Push lib file

adb: error: failed to get feature set: no devices/emulators found

16-3-2018_01-11-14 ===> Push lib file

adb: error: failed to get feature set: no devices/emulators found

16-3-2018_01-11-15 ===> Change file attributes

error: no devices/emulators found

16-3-2018_01-11-16 ===> Change file attributes

error: no devices/emulators found

16-3-2018_01-11-17 ===> Change file attributes

error: no devices/emulators found

16-3-2018_01-11-18 ===> Change file attributes

error: no devices/emulators found

16-3-2018_01-11-19 ===> Change file attributes

error: no devices/emulators found

16-3-2018_01-11-20 ===> Change file attributes

error: no devices/emulators found

16-3-2018_01-11-21 ===> Change file attributes

error: no devices/emulators found

16-3-2018_01-11-22 ===> Change file attributes

error: no devices/emulators found

16-3-2018_01-11-23 ===> Change file attributes

error: no devices/emulators found

16-3-2018_01-11-24 ===> Change file attributes

error: no devices/emulators found

16-3-2018_01-11-25 ===> Change file attributes

error: no devices/emulators found

16-3-2018_01-11-26 ===> Change file attributes

error: no devices/emulators found

16-3-2018_01-11-27 ===> Change file attributes

error: no devices/emulators found

16-3-2018_01-11-28 ===> Change file attributes

error: no devices/emulators found

16-3-2018_01-11-29 ===> Change file attributes

error: no devices/emulators found

16-3-2018_01-11-30 ===> Push XperiFIX Flasher

adb: error: failed to get feature set: no devices/emulators found

16-3-2018_01-11-31 ===> Push Magisk

adb: error: failed to get feature set: no devices/emulators found

16-3-2018_01-11-32 ===> Run Xperifix Flasher

error: no devices/emulators found

16-3-2018_01-11-33 ===> Flash Magisk

error: no devices/emulators found

16-3-2018_01-11-34 ===> Wipe Cache

error: no devices/emulators found

16-3-2018_01-11-35 ===> Wipe Dalvik

error: no devices/emulators found

16-3-2018_01-11-36 ===> Reboot

error: no devices/emulators found

What am I doing wrong?

BTW the Internal storage shows 0 MB in TWRP.