Error - Paypal did not complete the transaction

[binit13]

Hi,
Anyone guide me to integrate Payment module (sandbox) for developer so i check it.
i am doing virtuemart .Payment Method Name:    PayPal (new API).username Api and signature
all i do but i cannot understand how i fix this problem.

I have try but i got this error
"Error: Error - Paypal did not complete the transaction. Please try again in a little while.
Error: Failure in Processing the Payment (ps_paypal_api) "

I make 2 account in https://developer.paypal.com/ .

any guide me
thank you