Would you be so kind and add
oneplus3t: "android-armhf-64binder"
to https://cdimage.ubports.com/anbox-images/devices.json.
It will work for the oneplus3t too, if this entry is in the .json file. The devices are very similar. I have tested it with a modified anbox-tool (changed the .json url in anbox-tool to a http server of mine and used a modified devices.json).

@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!

oneplus3t: "android-armhf-64binder"
is missing in https://cdimage.ubports.com/anbox-images/devices.json. That's why it does not work the standard way.

pascal, pl/1, abap, sql, apl, c, java, qml. maybe some more.