Possible reasons of this error.
You have typo in domain name during site creation.
For your information Your server had by default apache server installed And its files like index page located at /var/www/html
If you will open this directory you will find two html files in it Index and index.nginx-debian
So default configuration file has instructions to show index page in case 404 etc but when you will visit your site by Ip then you will see content of index.nginx-debian
Hope this will help