In a Microsoft Lync Server 2013 environment, a user can't share the desktop during a conference. In addition, this issue may affect all participants of the conference. You could see the following issue message in the Lync Server event log:
No MCU is available via the MCU factory
This issue occurs because the affected user has a conference that's stuck in Application Sharing Multipoint Control Unit (ASMCU) in "Creating" state. This means that any add conference requests to ASMCU for this conference are piled up and never get a response because the assumption is that some threads are working on initializing the conference. However, the thread that's responsible for conference initialization has crashed with an unhandled exception.
To fix this issue, install the July 2018 cumulative update 5.0.8308.1001 for Lync Server 2013, Conferencing Server.
There's another issue may also cause a user can't share desktop in a meeting in Lync Server 2013. Please see, Fix the startup to initialize McuInfra components only after ASMCU is completely ready to take new requests in Lync Server 2013.