Unable to install Ubuntu 18.04 from 17.10

I’m trying to upgrade Ubuntu but there is no new release available.

Output of lsb_release -a:

No LSB modules are available. Distributor ID: Ubuntu Description:    Ubuntu 17.10 Release:    17.10 Codename:   artful 

Output of sudo do-release-upgrade:

Checking for a new Ubuntu release No new release found. 

Also, when I do Alt+F2 and update-manager -c it says “Failed to download repository information”.

Output of cat /etc/apt/sources.list:

deb http://archive.ubuntu.com/ubuntu/ artful main universe restricted multiverse deb http://security.ubuntu.com/ubuntu/ artful-security universe main multiverse restricted deb http://archive.ubuntu.com/ubuntu/ artful-updates universe main multiverse restricted 

Thanks for any help.