I have designed a website in Adobe Muse.
I try to upload the exported html to WordPress, but I get the message 'The theme has no style.css stylesheet'.
How can I solve this?
Uploading HTML to WordPress isn't going to work. WordPress is a program that stores your posts and pages, etc. in a MySQL database and it creates HTML pages as it's output. To some extent you can copy your HTML content into the backend of WordPress, but you still need to install a theme. That's the error you're getting. There's a product called MusexPress (musegain.com) that says it converts Muse into WordPress. I have no idea if it works, but it has a free trial so it's worth a shot.
You are not facing the same issue - Muse is under EOL since 2018 and since then not developed in any case except that it still works under certain circumstances. The widget "musexpress" will probably not work any more as the vendors moved on as well and so should you. Muse to WP is not a good idea anymore if it ever was.