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 Business Central. 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.
Here are some of the most common errors Lightyear receives:
The VAT Posting Setup does not exist. Identification fields and values: VAT Bus. Posting Group='DOMESTIC',VAT Prod. Posting Group='SERV NORM'
The Tax Code on a line of the document does not have a VAT Posting Setup within Business Central.
Information on how to set up your Tax Codes can be found here.