@oden Something is wrong before that. After you cd into Downloads then do
Sudo -o
and hit enter you shuld just enter the password for phablet and full steam ahead. None of that sudo unable to resolve host ubuntu-phablet stuff, but I have no idea what is happening, sorry.
The port version you are using is it the FP3 installer config or the FP3/FP3+ port - Hallium 10. No idea if that has the slightest to do with it but the info may help someone.