Users are reporting problems adding addresses to email in Outlook on the web. Microsoft is aware of the issue but I dont have any information on the cause or when it will be fixed.
Some users say it works correctly in Chrome browser and only fails if using Safari.
Update: check for updates for Safari and see if that fixes it.
IMAP Yahoo and AOL accounts might experience issues with re-syncing folder content - Microsoft Support
While we're waiting to find out the cause of the bug and for it to be fixed, you can open the compose message in a new window.
Click the button to pop it out into a new window and you will be able to click in the recipient fields.