That’s because you don’t have a form element in your HTML. You have a closing tag, though, but no opening tag: @ArielLeslie has better eyes than me, your form is there but the input isn’t inside the form:
<input type="text" placeholder="cat photo URL">
<form action="https://freecatphotoapp.com/submit-cat-photo">
<!-- no input inside form tag :-( -->
</form>
There’s an opening tag on the same line as the input.
@ily What do you mean when you say you can’t put it inside the form element? Right now you have it before the form element. On the other hand, if you look at the ul and ol elements, you will see that they both have li elements nested inside of them.