This usually happens when you try sending emails to target customers (email segmentation). This feature allows you to send unique emails to a specific group of customers and also add new email notifications targeting a different group. This feature is available for customers in the Scale plan and above.
To know how to create a custom segment Click Here.
You can choose to upgrade your plan if you need to utilize this feature or you can select "All invoices" (default) in the send to option on the invoice receipt email notification.
You could do this by navigating to Settings >> Configure Chargebee >> Email notifications >> Required email notification >> Edit >> Send to: . Here's a screenshot for your reference.
Feel free to contact Chartebee support for more info.