Is your debit card "co-branded"? (VISA/DEBIT or MASCTERCARD/DEBIT)
If so you will be required to use the credit card submission for an online payment. *More information click here

Generally, co-badged debit cards in market today cannot be used for making online purchases using Interac Online. Some financial institutions have chosen another payment network(VISA or MC) for online debit transactions, and consequently, their customers with a co-badged debit card will not have access to the Interac Online service. For more information about the services accessible with a co-badged debit card, contact your financial institution directly.