Before sending an invoice, the user needs to create a draft of the invoice. Please see this article on more details of how to create draft invoices.
Once the draft has been created, select the checkbox next to the invoice(s) you wish to send, and click the Send button from the top of the screen. The sent invoice will now appear under the Invoices>Sent screen.
In case of missing details required to send the invoice(s), an error message may appear.
Hover over the yellow error triangle symbol to receive an explanation for why the error occurred. As an example, the user may not have selected an invoice delivery method.
Alternatively, the user may have specified ’email’ as the preferred delivery method but failed to specify the recipient’s email address in the details.
Click on the Amount of the invoice to access details and resolve the issue specified by the system. Click save and try again.