How to redeem vbucks gift card?

My son got some vbucks gift card as Christmas gift. When we try to log in the Epic account to redeem, we have the problem of "Sorry the credential you are using are invalid" error. when we try to log in by using the xbox account, it says "there was an error processing the credentials from the external login system. Please contact support".

I have checked and sure my xbox and microsoft account are linked up. However I can't login by either way to redeem the gift cards. Could you pleased advise what can i do? thank you

:)