You didn't miss it, it wasn't released yet. So no need to worry.
Although it was added to the game in the Dragonhold patch, and you can already preview it ingame by traveling there and entering the house, it wasn't made available for purchase yet. There's often quite some delay between adding the houses to the game and actually being able to purchase them. Jode's Embrace for example was added in Elsweyr, but became available for purchase only during Dragonhold.
So keep an eye on the monthly Crown Store showcases, ZOS will definitely advertise the release of Potentate's Retreat like they did with all their new houses. You haven't missed it yet!
I noticed it just now, they already announced Potentate's retreat for this month! I guess since I'm not interested in the house, the release info just didn't stick in my memory lol. You'll find it in this December's Crown Store showcase:
'The Potentate’s Retreat home will be available in the Crown Store on all platforms for a limited time, from December 19 to January 2 at 3pm GMT. This house will be available in furnished and unfurnished versions.'
So sounds like you're in luck, it will be available soon!