When I installed Nordvpn on my windows PC it installed easily. In elementary I downloaded the package and tried to install from the terminal after Gdebi failed to install it. In terminal it got as far as reading packages but stopped with an "unsupported file" error message.
Please download the version of nordvpn and open a terminal window pressing Ctrl+Alt+T and execute the following commands,
cd ~/Downloads sudo apt-get install ./nordvpn-release_1.0.0_all.deb sudo apt-get update
Please note that we are changing directory to "Downloads" because this is where the downloaded files are stored by default and we will have to execute the two following commands from where the file is saved. If you manually saved the norvpn file somewhere else, please change working directory to that directory instead. For example,