Strange. It allows me to add the book to my cart and go to the checkout page. I didn’t progress any further than that because I already own a copy and don’t need a second.
Maybe it’s a territorial thing? Are you in the U.K.?
Phaidon emailed me yesterday saying they couldn't get the signed books or oversold and are sending an unsigned one and giving me a $20 coupon on a future order. they should have just cancelled in full.