Hi,

In Odoo, the "Send by Email" button on invoices only appears under specific functional conditions. It is designed to be visible only when the invoice is posted, belongs to a customer invoice or customer credit note type (out_invoice or out_refund), is not currently being sent, and does not already have a generated PDF report (invoice_pdf_report_id is empty). Functionally, this means Odoo expects you to send invoices by email only after they are validated and not through automated processes. If your database uses automatic invoicing or email workflows (such as through eCommerce, Subscriptions, or POS), Odoo often pre-generates the PDF and marks it as already sent, which hides the button. To test it manually, create and post a customer invoice that isn’t linked to any automation the button should then appear.

Hope it helps