This happens when the account you are adding your document to (normally supplier document) can't be used for input costs as the account you connected to is not set correctly in your Xero account. For example, Xero does not allow input costs to post to an account code that is not marked as an Expense account. This can result in supplier invoices failing to sync.
Troubleshooting
In Fergus, go to Settings, open the Integration Centre, and click Xero.
Under Account Settings, edit the account code to an expense account or choose the correct account by clicking on the drop down for Purchase Invoices and/or Supplier Credits.
In Xero, confirm that the intended account codes are correctly configured as Expense-type accounts.
Once done, head back to the supplier document then click on the 3-dot button to resend invoice to Xero.