If you wish to add a footer and you don't see it in a Block Library, it's possible your template already contains a footer. It is not possible to add another Footer block if there's is already another footer enabled.
Tip: Sometimes it's difficult to see a footer when it contains no navigation items or a logo.
To check if you already have a page footer:
1. Hover over the bottom of the web page.
2. Delete the existing footer by clicking on the trash icon.
Now you can add a new Footer block from the Block Library.