How to fix this problem?

How do I fix the fourth error mistake? Because all sections do have an <header> element and all they have the same name, just the Ids have “_” rather than spaces.
https://codepen.io/danambitous/pen/gOabEZe

Hello there.

It is a simple typo. The error message (bit in red) is very helpful.

Answer:

You have 2 instances of missing the letter “s” in the id of a .main-section

Thank you, how do you blur the answers like that?

Sure thing.

If you go to the chat you should have started with the discobot, when you joined the forum. Reply to the thread with this:
@discobot start advanced tutorial
You will be taken through a tutorial teaching you how to use a discourse forum.

Alternatively, you can search online how to use these features. Just look for MarkDown syntax.