There are MANY posts like yours here on the CC and the fact that you can't add notes anymore is due to the new release unfortunately. Everyone is hoping that Airbnb will put it back so it's really important to give them your feedback to push them along!
FYI, here is one of the links from an earlier post:
Yes, the ability to add notes on the regular calendar for open or blocked dates was removed in the last release (probably not intentional) For now, Hosts need to add notes by turning on Professional Hosting Tools and using the multicalendar to add the note. You can only access the multicalendar on the desktop/laptop.
The note is then viewable in the App and on the regular calendar.