@Kyrylo looks like that not all partitons are ext4. May be a clean start with the device can help you. It gives you best chance to do it.
- start with twrp (fastboot boot twrp)
- set twrp to adb sideload mode
- adb sideload the latest oxygenos 9.0.6 from oneplus support page
- start the phone and make initial setup of the os
- after that 'fastboot boot twrp' again
- 1st. wipe all partitions
- 2nd select wipe advanced 'change filesystem' of cache, system and data and set it to ext4
- 3rd wipe format data
- check with twrp terminal command 'mount' that all partitions are ext4. If not change filesystem again.
- connect device to usb (still in twrp) and start ubport-installer
But this is not an op3 specific question!
Hey, just wanted to say thank you as this helped me get past an error during the automatic installer.