If you would like to use Ubuntu Linux distribution in your AppVMs, you can build and install one of the available Ubuntu templates. These templates are currently not provided by Qubes in ready to use binary packages, because Canonical does not allow redistribution of a modified Ubuntu. The redistribution is not allowed by their Intellectual property rights policy.
To quickly prepare the builder configuration, you can use the
setup script available in the repository - it will interactively ask you which templates you want to build.
Then, in the qubes-builder directory, run:
make qubes-vm make template
The build for Ubuntu 16.04 LTS (Xenial) is straightforward.
The build for Ubuntu 18.04 LTS (Bionic) is straightforward.
If you want to help in improving the template, feel free to contribute.