Problem
Your customer is trying to use the online payment link, but it's not working.
Solution
The online payment link will not work for the following reasons:
The invoice associated with the online payment link has already been exported
The invoice associated with the online payment link is an adjustment invoice.
The Online Payment Link on the invoice email template is using square brackets instead of curly brackets. For example, your template is using [OnlinePaymentLink] instead of {OnlinePaymentLink}. Make sure you are using curly brackets {OnlinePaymentLink}.