@TeamYouTube There is a problem with "Add video to playlist". If you choose the URL tab and paste a youtube URL it finds the video but when you hit "Add Video" it says "The video you selected no longer exists. The owner may have removed it" which is incorrect. Were you aware?
Thanks for the tag. If the video still exists, it shouldn't display this message. Try clearing your browser's cache & cookies, then try adding the video again. If it doesn't work, try using a different browser or computer. Let us know if it works.