note that openvpn should be installed for this to work, install it with the below command if you didn't already:
sudo apt install openvpn
if it doesn't fix it try below steps
systemctl status nordvpnd gives us this handy error [Error] starting openvpn: fork/exec /var/lib/nordvpn/openvpn: exec format error
this occurs because of architecture difference that the openvpn version nordvpn installs and the openvpn version which should be installed