I'm no longer a Linux user; I've seen plenty of people pulling their hair out here on the forum when it comes to Ubuntu in combination with Arduino 
And I don't have a Pi.
I'm just throwing this in:
- Is your board recognised by the OS? lsusb should help you to figure that out.
- Does dmesg tell you anything?
- Uninstall brltty.
- As far as I know, Ubuntu comes with the CH340 driver.
- I've seen posts with something regarding udev rules but I have no idea what; do a search.
Why did you not stick with Raspbian? You might have very valid reasons, I'm just curious.