The Editor button only appears on certain themes. If you are using a Subscription theme - Moonstone, Obsidian, Amber, Topaz, Bullion - then you do not have the ability to edit the contents of the page with the Editor.
Transactional-only themes are a legacy feature and are no longer offered. Support for the code editor was discontinued at the same time as these transactional-only themes.
Currently, you can make modifications beyond what's available in the theme editor by utilizing CSS in the editor's Custom Styles box (Distribution > Platforms > Edit Theme > Custom Styles).