When a customer chatted through Facebook massager, The businesses will be given 24 hours to respond to the client. After 24 hours since a customer's latest message, the opportunity to respond will close. If Admin or Agents are trying to reply to it outside the 24 hours restriction, you might find this error:
We realize how important it is for you to keep connected to your client. At this moment our team is trying to find a solution to this particular issue.
In the meantime, please make sure that all the messages from your clients are replied to within 24 hours, you may want to try to reply directly from your Facebook Business Page or you have to wait until the client is sending another message.