The value of PAYMENTREQUEST_0_AMT doesn't equal the sum of the following: 

• PAYMENTREQUEST_0_ITEMAMT (required)
• PAYMENTREQUEST_0_TAXAMT (optional)
• PAYMENTREQUEST_0_SHIPPINGAMT (optional)
• PAYMENTREQUEST_0_HANDLINGAMT (optional)
• PAYMENTREQUEST_0_SHIPDISCAMT (optional)
• PAYMENTREQUEST_0_INSURANCEAMT (optional)
  

To resolve this issue, check that the sum of the values equals the value of PAYMENTREQUEST_0_AMT.

For example:

PAYMENTREQUEST_0_AMT=524.20                     // Total
PAYMENTREQUEST_0_ITEMAMT=458.00                 // item * quantity + other items * quantity = ITEMAMT
PAYMENTREQUEST_0_SHIPPINGAMT=20.00              // shipping total
PAYMENTREQUEST_0_TAXAMT=46.20                   // tax total
L_PAYMENTREQUEST_0_NAME0=Books
L_PAYMENTREQUEST_0_AMT0=154.00                  // 154.00 * 2 = 308.00
L_PAYMENTREQUEST_0_QTY0=2
L_PAYMENTREQUEST_0_NAME1=CDs
L_PAYMENTREQUEST_0_AMT1=50.00                   // 50.00 * 3 = 150.00
                               308.00 + 150.00 = 458.00 (same as ITEMAMT)

                                          20.00 // shipping

                                          46.20 // tax

                                         524.20 // AMT

See the Developer Portal for a complete list of NVP/SOAP API error codes.