Lightyear has a ton of built in validation checks to ensure the data is correct in Lightyear before trying to send the bill and information to iplicit.
However, there are occasions when the export fails and Lightyear displays the reason for the error - you will see a warning message with information like the below, directing you to view the document's Activity History.
Account <xyz> is not valid for purchases
An Account Code selected on the invoice/credit note line item has not been set up to use with Purchases within iplicit.
Solution: You can either send the document back a tab in Lightyear and choose another Account Code, or change the Account type to allow it to be used with Purchases in iplicit.