NordVPN on Android has recently switched to sending the user to a webpage to do the log in (as opposed to handling it natively in the app).
If your Android did not set a default browser, then it seems to open its own built-in browser to handle this process, which works.
However, if Opera Touch is set as the default browser, this overrides the native browser, so the login session is opened in Opera Touch.
However, after logging in, NordVPN tries to send you back into its app. This is where the incompatibility comes in.
Opera Touch was unable to open the link since no application associated with "nordvpn:" protocol was found on your system.