όταν εκτελώ την εντολή $ sudo apt-get install libbz2-dev libsqlite3-dev libboost-dev zlib1g-dev μου βγαίνει το εξής error:

Err:1 http://ppa.launchpad.net/x2go/stable/ubuntu xenial InRelease

  Could not resolve 'ppa.launchpad.net'

Err:2 http://gr.archive.ubuntu.com/ubuntu xenial InRelease                                           

  Could not resolve 'gr.archive.ubuntu.com'

Err:3 http://gr.archive.ubuntu.com/ubuntu xenial-updates InRelease                                   

  Could not resolve 'gr.archive.ubuntu.com'

Err:4 http://gr.archive.ubuntu.com/ubuntu xenial-backports InRelease

  Could not resolve 'gr.archive.ubuntu.com'

Hit:5 http://security.ubuntu.com/ubuntu xenial-security InRelease

Reading package lists... Done                                  

W: Failed to fetch http://gr.archive.ubuntu.com/ubuntu/dists/xenial/InRelease  Could not resolve 'gr.archive.ubuntu.com'

W: Failed to fetch http://gr.archive.ubuntu.com/ubuntu/dists/xenial-updates/InRelease  Could not resolve 'gr.archive.ubuntu.com'

W: Failed to fetch http://gr.archive.ubuntu.com/ubuntu/dists/xenial-backports/InRelease  Could not resolve 'gr.archive.ubuntu.com'

W: Failed to fetch http://ppa.launchpad.net/x2go/stable/ubuntu/dists/xenial/InRelease  Could not resolve 'ppa.launchpad.net'

W: Some index files failed to download. They have been ignored, or old ones used instead.

σε τι οφείλεται αυτό και πώς μπορώ να το διορθώσω; 

Ευχαριστώ