Chess960 Explained!
In Fischer Random you have to drag the King and drop it on top of the Rook to castle.
It should also work if you click the King once then you click the Rook once without clicking any other squares.
All chess castling rules apply: once only, not moved, not in check, not passing through check, Rook can be under attack just as long as it does not affect the King.
I looked at the game you are playing, you moved one Rook from c8 to d8 so now you can only castle with the Rook on a8 but the Rook on d8 is blocking that, you need to move the Rook on d8 before you can castle.
The final positions of the King and Rook after castling is exactly the same as chess.
https://www.chess.com/daily/game/179902434
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