Cannot unlock bootloader
In some devices you can enable OEM unlocking even with the option being disabled in the Developer options menu. Ensure ADB Debugging is enabled on the device and try this command

sudo -i
adb enable oem unlocking
adb reboot bootloader  

Then, once the bootloader is loaded use fastboot to attempt an oem unlock. Note that this still may not work as many devices require a code or unlock token from the manufacturer to actually unlock the bootloader and I can find no information of a successful bootloader unlock for this specific device.

Also be aware that Ubuntu Touch has been discontinued and your device does not seem to be supported anyway, so all this may be for nothing. There also does not appear to be any custom ROM's available for your device.