Hey gas-, thanks for reaching out and sorry about the issue you ran into there.
First and foremost, can I just double-check where you've attempted to buy this? Was it possibly through Ubisoft Store by any chance?
Please mind that content may not always be available in all regions therefore if it's unavailable in US Store at the time, then I am afraid you will not be able to purchase it. Additionally, I can see that it is available in UK Store therefore if you have attempted to purchase it through there, then it will lead to the error you've stated.
Apologies if this may not be the outcome you may have anticipated and if you have any further questions, let us know!
First and foremost, can I just double-check where you've attempted to buy this? Was it possibly through Ubisoft Store by any chance?
Please mind that content may not always be available in all regions therefore if it's unavailable in US Store at the time, then I am afraid you will not be able to purchase it. Additionally, I can see that it is available in UK Store therefore if you have attempted to purchase it through there, then it will lead to the error you've stated.
Apologies if this may not be the outcome you may have anticipated and if you have any further questions, let us know!