If you are unable to purchase the 'Choose Your Ascension' offer, it may be because you have not selected your preferred Epic Ascension Item. The purchase button will only appear on the offer once you have tapped on the Ascension Item you wish to choose.
Please note - you can change the Ascension Item you receive with the purchase by simply tapping on the desired Ascension Item.
Before choosing Ascension Item:
After choosing Ascension Item (Damascus Blade has been used as an example):