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 Suddenly I can't add my own videos to my playlists that are marked as series. The videos and the playlists are my own and both of them are on my own channel. This issue only happens with any of my playlists that I have marked as a series. This started 2 days agopic.twitter.com/zBI0vdqD6B
-
-
-
None of my videos that I'm trying to add to a series playlist are part of another series playlisthttps://youtu.be/MuCRMNeAbpA
- 1 more reply
New conversation -
Loading seems to be taking a while.
Twitter may be over capacity or experiencing a momentary hiccup. Try again or visit Twitter Status for more information.