Marble piece stuck in the sculpture

I'm assuming you are doing the Shadow Chess Piece fight for the first time?

If so, after placing all three of the Suspicious Marble Pieces into their respective statues, you need to mine it (with a pickaxe) on either the New Moon to do the Shadow Piece (bossl fight or a Full Moon for it to summon Clockworks.

Either way you do it, you'll get the Sketches to redo the fight again in the future.

I think they mean that a suspicious marble (the piece required to repair the sculptures) spawned inside a sculpture, making it impossible to reach the marble. Something like this: