I'd like to add a synopsis to The Autopsy of Jane Doe, but there is no option to add one. I don't see the "Edit Page" option described in the help article that I read.
The Edit button should appear at the bottom of the page in either way of viewing on imdb.com (you didn't specify how you were accessing IMDb: computer or the app).
You could have found this link by using your browser's search feature, ctrl-F, and typed Edit. In fact, Edit appears several times on the page in smaller links for various categories, one of which is the Storyline group.
If you are using Reference view https://www.imdb.com/title/tt3289956/reference you should get in the habit of switching to standard view to find things, using the Change View link provided: