When there is a lot of YouTube activity, sometimes YouTube search does not work. If this happens, please wait for a few hours and YouTube search comes back. 

If you have the direct YouTube link to the video you want to add, here are some alternatives to adding the YouTube video to your lesson:
1) Click the "link" icon

2) Paste the YouTube URL direction in the search bar and search for it

3) Drag the corresponding tile to your lesson