If you downloaded the
.deb via a browser it should be in
cd to that location:
sudo dpkg -i <name of file>.deb
If it shows broken packages or missing dependancies follow on-screen instructions.
In response to below comments, seems like the
.deb file just adds a repository but does not install the nordvpn package.So you need to run
sudo apt-get update
sudo apt-get install nordvpn
as well to actually install it which wasn't clear to me.
Check for these two lines for a successful addition when
apt update is run:
Get: https://repo.nordvpn.com/deb/nordvpn/debian stable/main amd64 Packages [3,298 B]
Get: https://repo.nordvpn.com/deb/nordvpn/debian stable/main i386 Packages [3,298 B]
Seems like the
.deb failed to add the contents to
/etc/apt/sources.list.d/nordvpn.list, so here are they
deb [arch=amd64] http://repo.nordvpn.com/deb/nordvpn/debian stable main
deb [arch=i386 ] http://repo.nordvpn.com/deb/nordvpn/debian stable main
sudo nano nordvpn.list
Add the above contents exactly as they are without any "#" at start or end in a blank line.
sudo apt update then and
sudo apt install nordvpn