If you're encountering issues with unsent email invoices and encountering the 'INVALID_FIELD_WHEN_USING_TEMPLATE' error in the Notes & Attachments section, here's what you need to know and how to address the problem

The presence of the 'INVALID_FIELD_WHEN_USING_TEMPLATE' error in the Notes & Attachments section indicates a potential misconfiguration in your email deliverability settings. To resolve this, navigate to Setup > Email Administration > Deliverability. Verify that the Access Level is configured to 'All Email,' as illustrated below.

If the previously mentioned configuration did not resolve the problem, kindly refer to the associated article for additional troubleshooting steps: Why Online Payments Fail To Send Payment Receipts