Most prepaid cards in Canada (e.g., Nextwave Titanium and Vanilla) do not allow online gaming transactions, which are coded as 7995. Ensure you use the correct card details and try with a slightly smaller balance. Most financial institutions will send back a generic error code, so it may be difficult for us to determine the exact reason your transaction was declined, so if the transaction fails, contact your prepaid card issuer's Support Team for more information on the rejection code.
As an alternative and if available in your MuchBetter app, you can head over to vouchermap.com and purchase a voucher with your prepaid card. You can then use this voucher to add funds to your MuchBetter wallet.