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 methods will impact your business reporting one way or another.
Consult your accountant or bookkeeper if you aren't sure how to proceed.