Why did my payment fail if I have money in my account?

Your debit order is linked to your bank card, not your bank account. Check that your card's online shopping limit is set high enough.
If you have a new bank card, update your card details on the customer portal. https://myaccount.sadv.co.za/register/ or contact us via WhatsApp 084 555 5585 option 2.
It might be an error on Vumatel's or your bank's side; retry the payment in the customer portal. https://myaccount.sadv.co.za/register/