That's unfortunate! I just called 866.792.3395 and picked the option for a new reservation at Miraval Arizona (I can't remember the order of the phone prompts). I was almost immediately connected to an agent who was able to help.

I see now that the number listed at the top of the BOGO promo page is different, but the number that I provided is provided in the small print of the Terms & Conditions:
Offer valid for stays completed between July 15 and December 31, 2020 at participating Miraval Resorts. Member must request Offer Code “WOH_Miraval_BOGO” at the time of reservation for a stay redeeming at least one Free Night Award. Additional free night must be used on the same stay. Reservations must be made in advance by calling 866.792.3395. Nightly Resort Credit will not be issued for free night awarded under this offer. Offer is not valid with previously booked or held stays or with groups/conventions and may not be combined with other promotional offers. Promotional blackout periods may apply due to seasonal periods or special events, and normal arrival/departure restrictions apply (unless otherwise noted). Additional charges may apply for additional guests or room type upgrades. Hyatt reserves the right to alter or withdraw this offer with a suitable substitute offer of equal value (if legally required) at any time due to actual circumstances. Hyatt®, Miraval®, World of Hyatt® and related marks are trademarks of Hyatt Corporation or its affiliates. ©2020 Hyatt Corporation. All rights reserved.