For the Concur Xero integration to populate account codes correctly, you will need to ensure that the mapping of the Account or GL Code from Concur matches a valid Xero account code.
If the solution has been configured to create invoices in Draft status, then the invoice will still be created, but without an account code set. It is always recommended that the mapping is correct though, as setting the account code in Draft mode before approval can impact on the Tax amount and tax adjustment that is typically set by the integration.
Tracking codes that does not map correctly will not prevent an invoice from being approved or cause an error within the application. It is recommended that regular reports are run to ensure that the configuration settings between concur and Xero does not cause transactions to transfer through without the tracking codes populated.