Why can’t I edit HTML in WordPress even with the administrator role?

You are using the WP Element plugin to control your content, as such HTML editing is disabled because the HTML source is basically impossible to edit by hand, as it contains all the format, content, blocks ( json ) and other configuration options which are stored together in the post_content.

Perhaps you can create new pages and not assign them to be edited with Element, this would then allow you access to edit the HTML.