Receiving error 10 is related to using Medallia Concierge's Facebook Messenger Integration. There may be one of three cases when you get "This person isn't available right now" message:

Case 1 : User has blocked you

Case 2 : You have blocked that user

Case 3: User has NOT blocked you (because you are able to see their profile mean that user not blocked you yet).

In last case, if you are able to see that user's profile but still unable to send them messages (i.e you are getting message "This person isn't available right now"), it will be probably when user has set privacy to messages they receive. If they have restricted messages they want to receive from by setting it in privacy AND they have deleted the conversation then you will get this message.

You will be able to send them messages until they do not delete conversation. Once they delete the conversation, the privacy act comes into play and you will no longer able to initiate the conversation, thus you can't send them messages first. Only that user can initiate the conversation after they've deleted the conversation.