Request Control in Teams if you want to access the screen. If you are unable to request or give control in Teams, see this post.

If Microsoft Teams Join button is missing or not working, confirm it with your Admin. You can also try clearing Teams cache or use these fixes.

Now, according to a posting on the Microsoft Teams UserVoice site, another tweak will add a 'join meeting' button in Outlook when a Teams meeting is added to your diary.