web hosting – How to Webhost – Beginner? (Error Message)

What do I need to do in order to avoid the message below?
I am hosting my first website site on GoDaddy. I did so watching the video here.

I ran a tiny bit of HTML as shown below:

<!DOCTYPE html>

I go to the domain name mathtyper.com that I recently purchased through GoDaddy, and I get the following message.

enter image description here