I assume that you know the edit function is under more in the upper right of your post and that something else went wrong ... but for others reading this thread ...
At present, occasionally some books that are either free or discounted are unavailable in the UK (and probably anywhere else outside America).
I can confirm your experience and also the unhelpfulness of the error message:
(the checkout system is working, for test purposes I just bought https://vyrso.com/product/14299/now-i-walk-on-death-row-a-wall-street-finance-lawyer-stumbles-into-the-arms-of-a-loving-god which is on sale)
EDIT: what I didn't check is whether your example book is really only on sale in the US and that's the issue. It's possible that there's a technical glitch for that resource - Brian or so would need to answer on that.
Andrew,
I apologize for the difficulty you're experiencing with purchasing discounted titles. I'll check with our development team to see why the promotional pricing isn't extending to you in the UK and see if the fix is something we can alleviate shortly. Again, I apologize for the problems you've been experiencing with the checkout function. Let me know if there is anything else I can do to help!
Thanks,
Brian