Not sure if it's still the case but you used to have to add a photo first, before adding a video.
Then, this is the process.
You can only add one video per listing, but you can use the same video on multiple listings - you just need to upload it once. Upload a video using the new listing tool by selecting Add video from the Photos & Video section: - The video should be .mp4 or .mov with a 150MB limit, or about one minute long https://www.ebay.com/help/selling/listings/creating-managing-listings/add-video-to-listing?id=5272
Use the Seller Hub and probably best to use a laptop / PC and try Chrome this seems to work well with eBay to manage this.
@jung_vi
