When is the button available
The “Issue invoice” button is shown only when you have an active connection to the accounting system and a classic invoice has not yet been issued for that order. If issuance fails, the button will turn red and hover will show the error message from the accounting system; you can try the action again later.
Billing settings in the accounting system
In the settings of the accounting system you will find switches for automatic issuance of invoices when the order status changes, after payment, or when the order is created. You can specify the invoice type (classic invoice, advance invoice), sequence, bank account, language, accounting templates and prefilled fields such as “Issue date” or “Variable symbol” using variables from the order.