I'm not sure if you mean you can't find where to reply or if the send message button isn't working?
When I go to my inbox, I see a list of messages. I then click on one, and the thread is topped by a blank box for my reply and just under it is 'send message' which is how I reply to their last message.
I am experienceing a similar problem. As a host, I received an email message from a guest about an inquiry but it does not show up in my Air BnB messages so I can not respond. The email is from Air BnB and it says to respond in 24 hours. I have tried a few times and refreshed my Air BnB page, etc. At one point I saw a message flash by but it was not up long enough to read carefully. The message was about the guest needing verification.
If a guest needs verification, whay are they able to inquire?