In order to embed web-based videos like YouTube videos in your presentations in PowerPoint 2016 for Mac, you'll need to install an Add-in for Office. Here's how:
Select the Insert tab on the ribbon, and then select Store. (Or, in your web browser, go to https://store.office.com.)
Enter "Web Video Player" in the search box, and follow the steps to install the add-in.
Once the add-in is installed, you'll be able to use it to play videos from YouTube or Vimeo directly in your slides. For more information, see Add a video to your PowerPoint 2016 for Mac presentation.