Why can't I email an invoice?
Check sending permission, the customer's email and the invoice email setting.
You need Send invoices permission, a customer email address and invoice emails enabled in the saved settings. The invoice detail page offers its email dialog after the draft stage and hides it for a void invoice. Ask someone with Manage business settings, roles & permissions permission to review the invoice email choice if it is disabled. Delivery can also fail after these checks. Follow Send an invoice and Configure email sending.