If you want to purchase the magical milestones key, it's better to turn on the gem/coins no cost and not consumed hacks, and then buying the gem version of the key (as this will cost you nothing).
As for the purchases being prohibited, that could be due to you using the signed version of the apk (I don't think that it can interact with the play store). I have spent money on the game before, though I didn't use the modded apk to do so. Instead I have my modded game on my pc, and logged into a non-modded install on my mobile to make purchases.