For adding a card to your Plivo account, you can go to your dashboard, and click on "Add a card"
Ensure to enter the exact card details as per your bank records.
In order to use complete Plivo services, you need to upgrade your account to standard. You could do so by doing a minimum recharge of $25.
You can set-up auto-recharge in your account to avoid manual recharge and constant monitor to your account credits. You can also set-up threshold amount from the drop-down, as per your convenience.
In case, the above is all clear and still the issue persists, contact our support team to assist further.