Thanks for sharing a screenshot, if a video is already part of a series playlist, it can no longer appear in another one. You can find more details about series playlists here: https://yt.be/help/Lces . Hope it clarifies!
@TeamYouTube
I can't add 2 of my videos to a playlist because it says that videos uploaded to another channel cant be added to a series playlist. The videos are on my channel. I have even changed the playlist to regular, added the vids and tried to put series on and it says:pic.twitter.com/g5cPVVF36A
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There is definitely a problem, videos aren't in any playlist and give this error when trying to add to one: Only happening on Private videos.pic.twitter.com/qPWoOQoHGK
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