If you wish to add a header and you don't see it in a Block Library, it's possible your template already contains a header. It is not possible to add another Header block if a header is already present.
Sometimes it's difficult to see a header when it contains no navigation items or a logo.
To check if you already have added a page header:
1. Hover over the top of the web page. Do you see the blue outline of the very top block on the page? Then the header has been already inserted.
2. Delete the existing header by clicking on the "trash" Delete icon.
Now you can add a new Header block from the Block Library.