During an online conversation or meeting, there are two possible reasons you might see this notification after you start sharing your desktop or an application:
One or more of the meeting participants are using an older version of Microsoft Communicator (now Microsoft Lync communications software) that does not support the viewing of shared content.
One or more of the meeting participants have not yet accepted your invitation to view shared content.
For people who are using older versions of Communicator, you can use other methods of sharing the information with them, such as file transfer, email, or IM. Otherwise, remind the participants to accept your sharing request.