It's not possible to preview unpublished videos in Composer articles. This is not ideal because we can't embed videos to preview them in the digital presentation. Unpublished videos (and still images, which is in another request) should be viewable when previewing an article in Composer.
T.J. Furman
Related to this idea as well: https://ideas.arcxp.com/ideas/GLD-I-117
Thank you for this idea submission. This is something that would require significant platform level work but it's certainly something we would like to support one day.