apt-get warning: No support for locale: en_US.utf8

Usually this error means that you could have been changing between different languages (locales) and something has caused this to error erroneously.

You could try regenerating your list of locales with

sudo dpkg-reconfigure locales

For me the result was:

Generating locales...
  en_AG.UTF-8... done
  en_AU.UTF-8... done
  en_BW.UTF-8... done
  en_CA.UTF-8... done
  en_DK.UTF-8... done
  en_GB.UTF-8... done
  en_HK.UTF-8... done
  en_IE.UTF-8... done
  en_IN.UTF-8... done
  en_NG.UTF-8... done
  en_NZ.UTF-8... done
  en_PH.UTF-8... done
  en_SG.UTF-8... done
  en_US.UTF-8... up-to-date
  en_ZA.UTF-8... done
  en_ZM.UTF-8... done
  en_ZW.UTF-8... done
Generation complete.

Then resetting your local locale with:

sudo update-locale LANG=en_US.UTF-8

i.e. use one of the Locale values in the output above

In my case, the problem was that /usr/share/initramfs-tools/hooks/root_locale is expecting to see individual locale directories in /usr/lib/locale but locale-gen is configured to generate an archive file by default.

I fixed it by running:

sudo locale-gen --purge --no-archive