There are two ways to set up a VPN connection with NordVPN on Ubuntu. The first one is to add NordVPN’s package repository to the apt sources and then install the nordvpn client via a simple sudo apt install nordvpn. The second one is to install OpenVPN and configure it to use the servers of NordVPN.
This how-to was written for Ubuntu 18.04 LTS (Bionic Beaver).

NordVPN setup with NordVPN package repository

  1. Download the NordVPN package (.deb) for Ubuntu:
    wget https://repo.nordvpn.com/deb/nordvpn/debian/pool/main/nordvpn-release_1.0.0_all.deb
  2. Install the package on your Ubuntu system:
    sudo dpkg --install ./nordvpn-release_1.0.0_all.deb
    Note that installing this package does only add the NordVPN package repository to apt, no executable is installed.
  3. Install the nordvpn client software:
    sudo apt update
    sudo apt install nordvpn
  4. Connect the nordvpn client (the server is selected automatically):
    nordvpn connect
    The general command to connect has the form:
    nordvpn connect [<country&gt;] [<city&gt;] | [server]
    so to connect to a specific country (and city) the additional arguments must be given:
    nordvpn connect United_Kingdom Manchester
    To get a list of available countries and cities you can query them with:
    nordvpn countries
    nordvpn cities
    It is also possible to give the server name directly:
    nordvpn de442

NordVPN setup with OpenVPN

  1. Install openvpn on your Ubuntu system:
    sudo apt install openvpn
  2. Get the OpenVPN configuration files for NordVPN:
    cd /etc/openvpn
    sudo wget https://downloads.nordcdn.com/configs/archives/servers/ovpn.zip
    sudo unzip ./ovpn.zip
    If an error is encountered during the download step above, it may be necessary to install the ca-certificate package:
    sudo apt-get install ca-certificates
    After unzipping ovpn.zip there will be a folder ovpn_tcp and ovpn_udp which contain the NordVPN server configuration files for OpenVPN.
  3. Connect to OpenVPN using one of the NordVPN servers:
    openvpn /etc/openvpn/ovpn_udp/de442.nordvpn.com.udp.ovpn
    You will need to enter your credentials for NordVPN.
Sources:
https://nordvpn.com/de/download/linux/
https://nordvpn.com/de/tutorials/linux/openvpn/
https://wiki.ubuntuusers.de/NetworkManager/VPN%20Plugins/