Without knowing the external program they are attempting to execute or what the options are, it's going to be hard for us to troubleshoot this. Have you reported it to nordvpn? Unfortunately we don't know enough from your error message to do anything towards troubleshooting it, aside from "an external program can't be executed".

This report has been filed against an old version of Kali. We will be closing this ticket due to inactivity.

Please could you see if you are able to replicate this issue with the latest version of Kali Linux (https://www.kali.org/get-kali/)?

If you are still facing the same problem, feel free to re-open the ticket. If you choose to do this, could you provide more information to the issue you are facing, and also give information about your setup?
For more information, please read: https://www.kali.org/docs/community/submitting-issues-kali-bug-tracker/