Following https://developer.ubuntu.com/en/phone/devices/installing-ubuntu-for-devices/
Model Samsung GT-S7392
Android 4.1.2
Android > Settings > Developer Options > USB Debugging = checked Android > Settings > Developer Options > USB Connection = MTP
$ sudo add-apt-repository ppa:ubuntu-sdk-team/ppa
$ sudo apt-get update
$ sudo apt-get install ubuntu-device-flash phablet-tools
$ adb reboot bootloader // phone reboots to normal screen !
$ fastboot devices
$ sudo fastboot oem unlock
< waiting for device > // Control+C
I've seen some talk of which phones this might work on, but nothing on the web page above. Should it work on my Samsung?
Samsung GT-S7392 is not supported, please read the instructions you are following, because at the second paragraph are links for supported devices.
https://developer.ubuntu.com/en/phone/devices/devices/