How do I fix the Xero error "Account code or ID must be specified?"

This Xero error typically occurs when an income account does not exist for one or more invoice line items.

To resolve: Select a default income account to use when one cannot be found on a product level. Go to Xero > Settings & Tools to select this, or make sure that you have specified a valid income account for each product in your Recurly data base. You can specify an income account under the accounting section of a product’s edit screen. Note: If you used a previous accounting integration with Recurly and you have old income account codes specified on products you will also receive this error.