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'm still having an issue where I can't add my newly uploaded private videos to my series playlists. I get this error:pic.twitter.com/tGywEsJ8Ln
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It is not part of another playlist and I have never had this problem before.
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