You installed it in an AppVM which won’t make it persistent. If you want packages to be persistent, you need to install them in TemplateVMs.
Well, clearly you didn’t because in that guide there’s no mention of installing proprietary vpn clients.
Method 1
Follow the guide that you linked (Using Mullvad VPN in Qubes | Micah Lee), which tells you to download the .conf (or .ovpn) files from the vpn website (in case of nordvpn: OpenVPN configuration files | NordVPN)
Method 2
If you want to use the nordvpn client, you need to install in a TemplateVM (for example debian-11, then create an AppVM, create bind-dirs and make the nordvpn configuration file persistent (/var/lib/nordvpn/data/settings.dat). In the AppVM:
$ sudo mkdir -p /rw/config/qubes-bind-dirs.d
$ echo "binds+=( '/var/lib/nordvpn' )" | sudo tee -a /rw/config/qubes-bind-dirs.d/50_user.conf
$ sudo poweroff
Then start the AppVM again, login with nordvpn login and then it should persist.
Method 3 (NOT recommended)
Otherwise you can create a StandaloneVM, install nordvpn there and use it normally, just be aware that this approach is less than ideal and NOT recommended since everything will be persistent.
qvm-create --standalone -t TEMPLATE -l red --prop provides_network=true --prop netvm=sys-firewall sys-nordpvn
(change TEMPLATE with the template you want to use, like debian-11 for example)
This topic has been covered multiple times and a solution for your problem already exists. Please use the search button: Search results for 'nordvpn' - Qubes OS Forum