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Learning to describe problems is hard, but it is an important part of learning how to code.
Also, the more you say, the more we can help!
Well, look very closely at your code here: <CatPhotoApp>Hello World</CatphotoApp><CatPhotoApp> is not a valid HTML tag.
The link that @hasanzaib1389 shared does in fact share the solution.
Where EXACTLY does it tell you to “substitute” h1 for CatPhotoApp? The instructions read:
Step 1
HTML elements have opening tags like <h1> and closing tags like </h1>.
The text for an element goes between its opening and closing tags.
Find the h1 element and change its text to:
CatPhotoApp
It says to “Find the h1 element and change its text to: CatPhotoApp”
So if you follow these instructions, change the text inside of the <h1></h1> from “Hello World” to “CatPhotoApp”