Assume that you have a topology where conferencing is disabled. This means that ASMCU and AVMCU should not be installed. When you try to apply patches to Microsoft Skype for Business Server 2015, you find OcsMcu.msp is not able to be installed.
This issue occurs because OcsMcu.msp tries to find the RTCASMCU and RTCAVMCU services that aren't present because conferencing is disabled on the topology.
To fix this issue, install the May 2017 cumulative update 6.0.9319.281 for Skype for Business Server 2015, Conferencing Server.