You can't join a meeting in Microsoft Lync 2013 (Skype for Business) when one of the following conditions is true:
You're an audio and video (AV) enabled user, and you try to join a meeting that has the audio or video feature disabled.
You're an AV disabled user, and you try to join a meeting that has AV feature enabled.
To fix this issue, install the June 7, 2016, update (KB3115033) for Lync 2013 (Skype for Business).
Note Lync 2013 was upgraded to Skype for Business in April 2015.