I would set defaults and walk step-by-step through this guide if that is not the course you took before. Quite often someone doesn't do something quite right and that provokes an authentication error. The server isn't forgiving. If everything is absolutely correct according to the guide and it doesn't authenticate, shut down Kodi, go to the Donation Checker, login and do an IPTV Reset and then restart Kodi.
Manual Guide Configuration
For Guide, Select one of the following for Preference: Prefer XMLTV, XMLTV only, Prefer Provider, Provider Only.
Select Cache Guide Data, and set CacheTime (hours) as desired.
If you select an external EPG other than **********, select Remote URL and enter the url for the provider.
If using the **********, select Remote URL and enter this url: http://nfps.**********
So what's the URL? Different setup from my last use of iptv66.