This happens only when the placeholders included in your invoice template do not have any value.
For example: When your invoice template has the placeholder "%Notes%" but you have not added any notes to your invoice, the invoice will not be sent. You will also receive a mail with the list of placeholders which do not have a value and needs to be replaced with a value.
To avoid this mail and to send the invoice directly to your customers, please remove the unwanted placeholders in your customized invoice template. Below, are steps to help you remove Placeholders.
Note: To know more about editing placeholders, click the video icon near the Invoice Templates link or click the Watch how you can customize your invoice template link that is on the right-side of the screen.
Zoho Invoice FAQ