I try to add a footer to my Map Series story.
Alexis Sancho Reinoso

Alexis — The HTML editor can be used for additional formatting of the content in the narrative panel of the story and for some basic CSS overrides of the page elements, but it can’t be used to add new sections to the page or make significant structural changes to the page. If you want to do that you can either embed your story map in another web page that has a footer or you can look to download the open source code for the story map template you’re using (map Series), customize the source, and host your customized version of the template on your own server. Here are some docs to help with those things if you’re interested in pursuing them.