If you are unable to send messages, it may be due to any of these 3 reasons:
First scenario: You will not be able to send/receive messages to/from a connected user, if the connected user blocks you or you block the person.
Second scenario: You cannot send a message to a connected user if the user removes your connection.
Third scenario: If the user disables this privacy feature then you will not be able to message ‘Allow you to send and receive messages from people who are not on your network‘.
Learn more about Blocking here.