Here are some of the PayPal error code which you should correct immediatly

Error:10411-This Express Checkout session has expired. Token value is no longer valid

Reason –  It means the user waited long to reach PayPal or make Payment. You need to again show the checkout page.

Error:10409-Express Checkout token was issued for a merchant account other than yours

You are using API credentials of some other account or the API credentials are incorrect.

Error:10434-Value of Custom element has been truncated

You are using custom elements while making express checkout call and the length exceeds permitted value

Error:11607-A successful transaction has already been completed for this token

You are processing the same TOKEN which is already completed. This happen when customer hit back button while making payment. There can be other reason where you might redirecting customer for payment for SAME token again.

Error:10472-CancelURL is invalid

You supplied incorrect value in CancelURL

Error:10410-Invalid token

Some Issue in your name value pair or token is expired

Error:10419-Express Checkout PayerID is missing

Error:10605-Currency is not supported

Error:-    PayPal Error