Suggestions:
As print invoice requires the invoice number when is not available in Sales order hence users cannot get the print invoice option on Sales order.
However, when users click on more options in the process trail link in the side panel of a closed Sales order record (which is already posted) in the feed view home screen , then in the process trail (side panel) on the corresponding despatch details. Users can get the invoice printing option.
Once the user clicks on the ‘Print Invoice’ option then the system opens the record criteria and users can get the preview or download the required report.
Refer in the below video of Despatch, Invoice creation and Invoice printing.