Chess960 - why can't I castle here

Game: lichess.org/tql3j7Ot/white

I try to castle king-side on move 10 by moving e1-g1 but the move was rejected (client-side) . Eventually I played c6 instead. My understanding of the rules was that this should be possible?

I tried again on move 11 and it was also rejected the same way.

Short castling indeed is legal there, but you have to move it as e1-h1 i.e. moving onto the rook. The reason for that is as follows: Imagine your king started on f1 instead. Then f1-g1 could just be a normal king move. So to avoid that problem you have to play KxR instead.

OK, thanks