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
nordvpn installs openvpn for: x86-64
you can verify this with: file /var/lib/nordvpn/openvpn (openvpn path for nordvpn might be different at the future, verify the path from the error)
outputs(trimmed unnecessary output): /var/lib/nordvpn/openvpn: ELF 64-bit LSB shared object, x86-64
raspberry pi works with: ARM aarch64
you can verify this with: file /usr/sbin/openvpn (your openvpn path might be different, execute which openvpn to find it's path)
outputs(trimmed unnecessary output): /usr/sbin/openvpn: ELF 64-bit LSB pie executable, ARM aarch64
to solve this we simply copy the correct version of openvpn to the nordvpn's openvpn path, overwriting the false openvpn version:
sudo cp /usr/sbin/openvpn /var/lib/nordvpn/openvpn (again, these paths might be different for you, verify them before proceeding)