Paypal Invalid Merchant Configuration sandbox error

I am using paypal pro in sandbox. when I send the request and in response I get this error:

Array ( [TIMESTAMP] => 2010%2d11%2d23T09%3a21%3a12Z [CORRELATIONID] => 2aa783b64659 [ACK] => Failure [VERSION] => 51%2e0 [BUILD] => 1620725 [L_ERRORCODE0] => 10501 [L_SHORTMESSAGE0] => Invalid%20Configuration [L_LONGMESSAGE0] => This%20transaction%20cannot%20be%20processed%20due%20to%20an%20invalid%20merchant%20configuration%2e [L_SEVERITYCODE0] => Error [AMT] => 20%2e00 [CURRENCYCODE] => USD )

I google this error and it is something with the "Billing Aggrement"

Can anyone suggest what to do on this?