Consider the following scenario:
User A starts a call with User B in Microsoft Skype for Business 2016 and User B accepts the call.
User A tries to transfer the call to User C who is offline.
If User C doesn't accept the call, User A receives a "Cannot complete the transfer" error notification.
User A clicks the error notification to see the error's details.
The error's details shows "Call was not transferred because User A cannot be reached and may be offline."
In this scenario, User C should be shown in the error's details.
To fix this issue, install the February 6, 2018, update (KB4011662) for Skype for Business 2016.