There are 2 paths you can take when creating an Invoice in Productive.
Billing > Invoices
Any Project > Invoices tab
Billing > Invoices

The Billing > Invoices screen shows all of your Invoices that have been created on all projects.

If you want to create a new Invoice from here:
Click the orange "+" button at the top right
Choose the Project
Choose a Budget
Click Create invoice
Any Project > Invoices tab
You can create an Invoice for a specific project while on that project's dedicated page in Productive.

You won't have to select a project after clicking the "+" button. Simply select a Budget and create the Invoice that way.