Google Wallet : No forms of payment available.

Google Wallet : No forms of payment available.

I'm a new developer for Android apps and some apps are not free. 5 people
have bought my app till now. Google Wallet notices that "You have no valid
forms of payment available. This account has no valid form of payment.
Please specify a form of payment in order to get paid". But when I clicked
in "specify a form of payment" then I got the message "No forms of payment
available". So I wonder: Is it normal? How much is the minimum payout
amount in order for me to be eligible for a payout at the end of my
payment circle? I'm Vietnamese. Thanks for your attention !