Occasionally, you might encounter an error when submitting details to Xero.

For example - Error message : The request was rejected by Xero.  Xero said: the request cannot be fulfilled due to bad syntax, invalid data, or missing/incorrect references.  Please check the submission logs where further information may  be included.

Similar to this 

Click the Errors button to view the logs. If you scroll to the bottom, you should find more details from Xero, such as any issues with a nominal account (for example, if it’s been archived or is inactive).

You will need to correct this in your Mappings and then the submission will successful.