I have been stuck on it for some time
Have you read the lesson text that describes what a self-closing tag is?
Briefly, it is any tag that does not have an ENDTAG, only and OPENTAG, and can thus not contain anything.
<p>This is a container element with opentag and endtag.</p> ^ ^ OPENTAG ENDTAG
Example list of self-closing tags
<hr> or <hr/> or <hr /> <br> <link> <meta> <input> <img>
There are others but i think you get the idea. Re-read the lesson and be sure you understand and follow the instructions.