Approved sales order needs to be accepted and create invoice after it.
Here is a reference video:
Step by step instructions to do this,
Goto Order Management from the left menu
Click on Sales order option
It will open up the existing sales order view page
Click on button to accept
Click on Ok button to confirm the accept
Type message for confirmation
Open sales order view
Click on icon to Convert to invoice
It will open the invoice page as shown below.
Here also you can tag your predefined template from the Template section although the billable items will be already added under Parts section.
Select the payment method and set the amount then Save the invoice or click Save and send Payment link as shown below.
