To see the payment methods that you can use with your Apple ID [account], click here --> Payment methods that you can use with your Apple Account - Apple Support
Note that these are ones that are accepted, but specific kinds (e.g., credit or debit cards) might be required in particular circumstances.
It may also help if your card comes from an institution in the country to which your account is set. Apple does not support using accounts set to countries where you do not actually reside.
Apple is checking with your financial institution if the payment method is valid and receiving a negative, or no, response. Make sure the name and address are entered exactly as it appears on your financial institution bills and that the 3 digit CVV is correct. Check with your financial institution about the status of your card.
If you want to speak with Apple, refer to this page for Apple Support features ➔ Contact - Official Apple Support
Select from the presented options until you find a solution for your issue, or see if there is a chat or phone call contact method offered lower on the page (you sometimes have to narrow down the options multiple times before this is shown). If you do not see your issue, keep experimenting with any series of selections until you reach one that offers a chat session or a telephone call and get the representative to redirect you.
This support article has various country telephone numbers for contacting Apple for support and service ➔ Contact Apple for support and service - Apple Support
You also say something about a MFI portal. I have no idea what that is.
