In order to embed web-based videos like YouTube videos in your presentations in PowerPoint 2016 for Mac, you'll need to install an Office add-in made by a company other than Microsoft. Here's how:
In your web browser, go to LiveSlides.com and follow the instructions on that site for downloading LiveSlides.
Drag the LiveSlides icon into your Application folder.
Now you are ready to embed a web video in your presentation. See Add a video to your PowerPoint 2016 for Mac presentation for instructions.