The IM button isn't enabled for all users in the contact card in Skype for Business 2015 (Lync 2013)

Symptoms

When you try to send the an instant message (IM) to a contact by opening the Contact Card in Microsoft Skype for Business 2015 (Microsoft Lync 2013), you find that the IM button isn't available.

Resolution

To fix this issue, install the April 4, 2017, update (KB3178731)  for Skype for Business 2015 (Lync 2013).

Note Lync 2013 was upgraded to Skype for Business in April 2015.