Authentication or 'auth' failures on Windows happen for a number of reasons, either relating to the password itself or the configuration of the application.
The most common cause of the issue is that the password is wrong. In this case, you need to change your password, log out from the NordVPN app and then log back in.
Change your password here: https://my.nordaccount.com/reset-password/.
If that does not help, reinstalling the application should:
- Go to Control Panel > Uninstall a Program / Programs and Features.
- Locate "NordVPN", "NordVPN network TAP" and "NordVPN network TUN" instances and remove them if they appear on the list.
- Restart the computer.
- Go to https://nordvpn.com/download/, download the NordVPN app, and go through the setup steps.