Here are some of the most common errors Lightyear receives, presented in [brackets]
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 Xero. 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 the document's Activity History.

Lightyear is trying to write Purchase Data to a Contact that doesn't exist in Xero. First you want to try to reimport the Contact from Xero following these steps:
1. As an admin, navigate to Settings > Accountancy > Supplier Cards.
2. Select the Contact or Supplier from the list, and delete it using the Trash Icon (don't worry, we're going to re-import this from Xero, if it exists) - If there are duplicate Supplier Cards please delete all of them (again, we are going to pull this info back from Xero if it exists)
3. Click 'Import all'
If the Supplier or Contact appears in the list, great... assign this as the 'Accounting Supplier' and you're good to export.
Xero Export failed: Tax rate must be Active
In markets outside of the US, UK, Australia and New Zealand, Xero has a series of default tax rates that are supplied "out of the box". One of these is "Tax on Purchases" which has a invisible label behind it (something you and I don't see) called "INPUT".
You, as a user, can set that tax rate from the default 0% to the tax rate of your jurisdiction. Lets for the sake of argument say you are a South African company, therefore you should set it to 14%.
When Lightyear sends data to Xero, the way we tell Xero that an that an expense carries tax is by telling Xero that the value needs to have "INPUT" applied to it. Xero then knows that "INPUT" relates to Tax on Purchases, and that Tax on Purchases has been set by you to be 14%.
But, Xero allows you to add or delete tax rates.
If you delete a tax rates such as "Tax on Purchases", you cannot reinstate it. From a Xero perspective it is now permanently inactive. If you delete it, you also delete 'INPUT" from Xero's records.
If Lightyear sends to Xero that an expense needs to have INPUT applied to it, but you have deleted INPUT from Xero's memory, the import will fail. In effect Xero says "I have no idea what INPUT means".
So in short, DO NOT DELETE THE DEFAULT TAX RATES THAT XERO PROVIDES FOR YOU
Please check that an Accounting Supplier card is linked to your Lightyear Supplier profile.
Head over to Suppliers > Select the supplier > check that an Accounting Supplier is selected.
If you can't find your supplier card here, you can create the Supplier Card within Lightyear (check that you don't have one existing in Xero first to avoid duplicates).
Update your settings and try to export again.
Xero Export failed: Account Code 'XXX' is not a valid code for this document
The Account Code/GL Code you have chosen for the bill cannot be used with purchases. When Lightyear sends bills through to Xero, Xero expects you to assign an Account Code that relates to purchases. i.e. expense code vs income code.
From the Signed Off tab, click MORE > Return to Review to send the bill back to the previous tab where you can assign a and expense Account Code to it. Then sign it off again, then try to export again.
Failed to export invoice: The Tax Type code 'OUTPUT2' cannot be used with account code 'XXX'
The Tax Code you've chosen for the bill cannot be used with purchases. When Lightyear sends bills through to Xero, Xero expects you to assign a Tax Code that relates to purchases. i.e. expense code vs income code.
From the Signed Off tab, click MORE > Return to Review to send the bill back to the previous tab where you can assign an expense Tax Code to it. Then sign it off again, then try to export again.
Xero Export failed: The document date cannot be before the period lock date, currently set at DD-MM-YYYY
You are trying to export bills with a date that is in a date range of a locked out period within Xero.
In Xero - unlock the date period and the export from Lightyear will now work.
If reporting is already done for this period, some customers will choose to change the invoice's date instead, but this is a personal choice and both may have negative consequences.
Failed to export Bill: [You have reached the limit of invoices you can approve]
This is an error message that Xero are sending back. This indicates that your Xero subscription level may have a quota on the number of transactions you can process.
You may need to consider upgrading your subscription in order to proceed.
Error: 'Organisation is not subscribed to currency'
Xero is sending this error message because your Xero account/organisation is not setup to handle foreign currency.
You have 2 options to resolve exporting this invoice and which one you choose is dependent on whether you are likely to use foreign currencies regularly;
Option 1:
Subscribe to foreign currency within Xero, then select that specific currency for use in Xero. This link below explains that you'll need to start a Business Edition pricing plan with multi-currency, or a trial organisation, to add transactions in different currencies. Only the Subscriber for your organisation can change pricing plans.
Option 2:
You can change the currency on the invoice from the Review tab or Approval tab, to your local currency - but make sure you edit the price as well after you convert the currency
Failed to Export Invoice [The line total [xxx] does not match the expected line total [xxx]. The total for this document must be greater than or equal to zero.]
You have manually edited an invoice, that's using a regular service/line item map, to be a credit note, and Lightyear is expecting the totals to be a positive number, or vice versa.
Send the invoice back to processing and manually edit to show the expected values, or search for a credit note map. If none are available, request a new credit note map.
This error is being received as the supplier has been archived in Xero and must be restored before the export will be successful. This must be done within Xero.
In Xero, go to your contacts menu and select All Contacts. Next, select Archived. This will present a list of all archived suppliers within Xero. Select the contact to open their details and click restore. Click restore to confirm.
For reference, see Xero help article: