When creating customers in Lightspeed Retail (X-Series), you have the option to set a limit on the customer's account.
Every time you create an Invoice for a customer, the amounts of these are added to their account both in QuoteMachine and in Ligthspeed Retail.
Although you will be able to create them in QuoteMachine, when the default or set limit for a customer is reached, you will be unable to sync them with Lightspeed Retail until payments are made to their account or the limit is increased.
Here are instructions on Setting or editing customer specific on account limit from the Setting on account limits article found on Lightspeed Retail's (X-Series) Support Website.
Once payments are posted to their account or the customer account limit is increased accordingly, you can manually export the Invoice to Lightspeed Retail.
Open the Invoice
Click the 3-dot menu at the bottom
Click Export to Lightspeed
The sync may take a couple of minutes. You will know it's successful when the activity Exported to Lightspeed with a link to the Sale on their website is posted on the QuoteMachine Invoice.