Let’s focus on the flashing problem first.
According to your logs, your problem (“Could not find a device online” in flashing Xavier A) looks the same as the other topic.
Besides replacing with another USB 2.0 Type A to Type A cable, could you also try with anything related to USB connection? e.g. connecting to another port on your host system, use another host system.
Also, please confirm if you use native Ubuntu 18.04 host system, not vm one.