In order to share Youtube videos with students in your lessons, they first must be marked as "approved" on Youtube. It they are not marked as Approved, the student will get a message that the video has been restricted.
You can approve individual videos by clicking on the Approve button that appears just below the video window (see first image below.)
If you are using a series of videos from a Youtube creator, you can approve their entire channel. You can approve the channel by clicking the Approve button on the channel landing page (see second image below.) If you approve a Youtube creators channel, then all of their videos will be available for students to watch. Use caution if you are approving a channel and make sure that the creator's content is appropriate.
Approve Video
Approve Channel