First of all, make sure you have your NNID set to US, region set to Oregon(or other sales tax free zone in US), then type in any real zipcode in the state you chose(you can find it easily with Google).

Then just launch your eshop, navigate to add funds, then choose the amount of funds you want to add to your account, then just type in your credit card/debit card info.

Previously, US eshop does not accept credit/debit card from outside of US, but they changed the policy since June 2017(or february??). I just tried it few days ago and it worked.

So you are now able to access the US eshop? The debit card is invalid or something, this could be due to last year or so, the bank negara had set a new rule where the bank should not opt-in the online purchase option for the account holder. Which mean you need to manually opt-in ( which is turn on) the option by yourself whether at ATM or your maybank2u account (i think it should be available there as well). If you already opt-in the option, then it is better you contact your bank. If you don't know exactly how to opt-in, you can contact maybank also (i also not sure since im not using maybank). Similar problem like your happened to me before and after opt-in, the card works just fine but i still can't make some oversea payment (some more setting have to deal with i guess).