Simply because IBE is not working anymore, sorry for that.
Right. Muse is discontinued and as of July 31, 2020 sites published using Adobe Muse will no longer be editable with Adobe In-Browser editing solution. InBrowser Editing is gone.
Well, frankly speaking, since 2018 Muse is under EOL, announced from my memory 2 years before, with the following announcement, as you may see in the post from Nancy, that IBE will be discontinued as well, you should have told your client quite early that there will be something in the line to be considered sooner or later. So from now on you may find a solution with your client to do necessary edits for them for whatever kind of subscription or no subscription and start a new project with another app if you are not into coding anyway.
1. We don't make the software. We are forum volunteers and product users like you.
2. You had more than 2 years notice that Muse was being discontinued. What did you think would happen if you didn't migrate your clients to other solutions?
Brackets is another open source code editor you could use but Adobe is ending its development on September 1, 2021.