Invoices can be generated automatically by enabling a scheduler in your org. The Salesforce administrator at your org will enable the scheduler to generate invoices by following the instructions here.

The invoice module is designed to generate invoices solely for time details that have attained the "approved" status from the previous month. Notably, this means that time entries from the current month, even if approved, are not included in the invoice generation process. 

Note: Please make sure you add the "Hourly Loaded Cost" and "Hourly Billing Rate" in the TT user screen for the user

Add the "Billing Rate" and the "Project Hourly Rate" in the Projects object. Only then do the invoices get created in your org.