Hi @dev02ali,
The way our free plugin creates an invoice automatically is when you select order email notifications in which it will be attached, under WooCommerce > PDF Invoices > Documents > Invoice > Attach to: The attachment process will create an invoice document with its respective number:
In the Professional extension, there is another way to create documents, based on the order status (without the need to attach it), specifically the Create automatically for setting. That is, if you select the Processing or Completed order status for this setting, the PDF document will be created only when the order reaches any of these order statuses:
In brief, if you are using only our free version, make sure you have selected the email notification(s) in which you want to fire the automatic PDF invoice creation.