Library says "Could not remove this video. Try again."

The video was not taken out of the list. It is still there. Try again. Use the more button (three dots) on its row, or slide the row to the left and tap Remove.

Check your Wi-Fi or mobile data first. SkipTube sends one remove at a time for each row, so a second tap while the first is still running does nothing.

Some rows have no remove item. Playlists and Liked cannot be changed here. Some rows in YouTube's history have none either, because YouTube gives no way to remove them. Change those on YouTube.

See Your Library.