sudo: unable to resolve host
If you ever get a message like, sudo: unable to resolve host, this may be if you changed your hostname with hostname -v newhostname or any other reason I'm not aware of.
Anyway, open /etc/hosts and make sure your new hostname matches the one in there. Your old one may be in /etc/hosts and needs to be manually updated.
Since you can't use sudo to do this, you will have to become root. Hopefully if you're using Ubuntu you enabled your root account, otherwise I'm not sure how you'll do this. See Enabling root on Ubuntu below.
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