Try to contact the support to see if it is a temporary problem. Since on their page, they wrote that the Visa debit card is accepted.
Go to this pagehttps://fizz.ca/en/contact-us, if you are not connect login
The business hours: every day from 8 a.m. to 9 p.m. (EST).
Wait 5 seconds for the bubble icon to appear at the bottom right of the browser and click on the icon to chat with support. Try a different browser if you have issues getting the bubble to appear.
If the chat bubble does not appear, this means that the number of sessions waiting for our answers surpasses our processing capacity. If that happens, send us a private message via Facebook Messenger or Twitter.
Your can also send a message via facebook messanger https://facebook.com/fizzca and click send a message (this is a bit slower)
You can also fill a contact form from your account Once logged in, from the account panel go to My Profile → Contact Form (the subject are limited) and fill out the form and someone will email you back.
A possible explanation could be if the address tied to your debit card is not in the Fizz standard coverage area of the province of Québec and Ottawa. Fizz apparently makes sure your address is in the coverage area before allowing you to sign up for cellular service.
You cannot pay with a regular Debit card. You can ask at your bank if they can switch you to VISA DEBIT card.
As stated by Fizz, you can use a Visa or Mastercard credit card, or a Visa Debit card to pay for your Fizz plans.
More info from Fizz here: https://fizz.ca/en/faq/what-payment-methods-can-i-use