I don't think the MyMcD application allows use of Canadian debit cards, which can't generally be used online [0]. I think it only allows credit cards—I've tried adding a credit card to take advantage of a deal, but the app is so terrible that I gave up after 15 minutes.
[0] Canadian debit cards are secured through chip and PIN, and the number on the front isn't a secret. You can use things like online bill pay or Interac e-transfer (which is not really used by businesses), and some banks allow you to create a virtual Visa card that's attached to your chequing account, but debit cards themselves are physical tokens that can't be used online.
It took several tries but many years ago I got bank of America to issue me a real debit card that couldn't be run without a pin (no visa logo). I haven't had success with any current banks, I assume because the visa mode is more profitable.
[0] Canadian debit cards are secured through chip and PIN, and the number on the front isn't a secret. You can use things like online bill pay or Interac e-transfer (which is not really used by businesses), and some banks allow you to create a virtual Visa card that's attached to your chequing account, but debit cards themselves are physical tokens that can't be used online.