NordVPN uses a certificate, which is signed by a qualified Certificate Authority (CA), for instance - signed by GoDaddy. If you are faced with the invalid security certificate error message, you are not reaching the real NordVPN server, and either your ISP or your network administrator is attempting to perform an eavesdropping or man-in-the-middle-attack. It's also possible that you have network monitoring software that is intercepting your network traffic - in that case you can try disabling it.
We have a blog post that explains web certificates in more detail here: https://nordvpn.com/blog/what-you-should-know-about-web-certificates/.
Here's how your screen may look with the error message:
Investigating further, we can see that it's clear that the certificate details have changed, since it's being interfered with. In this case, it was a Cisco firewall: