Review the suggestions in this link about what to do "If your payment method is declined in the App Store or iTunes Store" Click here --> https://support.apple.com/HT203005
Note if you are using the Canadian Store you will need a valid payment method for Canada which will likely need a billing address for you in Canada.
If you still have a problem contact Apple:
Click here --> https://support.apple.com/choose-country-region/media-services Select an option and keep on working your way down the menu until you get to one saying your "topic isn't listed". This will present you with support by chat or telephone call.
or
Contact Apple for support and service --> "See a list of Apple phone numbers around the world." Click here --> https://support.apple.com/HT201232