The most likely reason is that your credit card company is declining the transaction because we are a Canadian company and you don’t typically buy outside of your country. You can call your credit card company and ask for the transaction to be approved or you can make your purchase from our Teachers Pay Teachers store. TPT is a large American company so the transaction will not be declined.
Here is a link to the Games Subscription on TPT: https://www.teacherspayteachers.com/Product/3508359
If you purchase through TPT, after you make your purchase, download the product PDF from your TPT account and follow the instructions to setup your subscription.