[Code:5000] [Error]XML Parsing Error at Line 1, Column 2579: cvc-maxLength-valid: Value '__________' with length = '43' is not facet-valid with respect to maxLength '40' for type 'FortyStringNotNull'."

Amazon might return an error similar to the following:

It's important to note that Amazon might have character limits on certain fields. Those character limits cannot be altered through any other platform. If you ever encounter an error like this, refer to the Amazon Seller Central Help documentation or the Amazon MWS/SP-API field specification to find the field that is giving you an issue.

For the example above, an MPN of 43 characters was input but since Amazon has a character limit of 40 for MPN, this error was returned.