In a Microsoft Skype for Business Server 2015 environment, you join a meeting by using Skype for Business Web App through Internet Explorer browser. When you select the Install Skype for Business Web App plug-in option, the following warning message pops up:
"With the plug-in, you can present your screen in the meeting. You can't participate in video of the meeting or join meeting audio from your computer."
To fix this issue, install the May 2019 cumulative update 6.0.9319.548 for Skype for Business Server 2015, Enterprise Web App.