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 f 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 a page header:
1. Hover over the top of the web page.
2. Delete the existing header by clicking on the trash icon.
Now you can add a new Header block from the Block Library.