There are two existing threads about this to review:
Long story short: A TinyMCE API key should not be required, and if TinyMCE asks for it, it indicates that something is wrong. Make sure your browser cache has been cleared, just in case that’s part of the issue, then have a look through the other forum posts to see if e.g. you can identify a Javascript error or a dependency that can’t be fetched by the browser.
Regards,
Alec Smecher
Public Knowledge Project Team