How are you trying to do it? If you click the king then click g1 does that not work? Or click-hold the king and drag it to g1?
Also, if you have any plugins, those could interfere with the code and cause problems.
Unable to castle?
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For some reason, the chess board won't allow me to castle. I've tried castling on both Windows edge as well as Chrome, but it does not seem to work. I've cleared my cache on both browsers as well. Castling works fine on my phone, but the desktop version is giving me problems. Attached is an image of a board that will not allow me to castle. Any ideas?