Why can't apt resolve names?
Solution 1:
Run this command from terminal and you could use apt and apt-get normally in the future :)
echo 'Acquire::ForceIPv4 "true";' | sudo tee /etc/apt/apt.conf.d/99force-ipv4
Solution 2:
If /etc/atp/apt.conf exists, make a copy of the file and then edit the original to comment out the following line:
Acquire::http::Proxy "http://proxyaddress:proxyport";