Looks like there's no way to make invoices directly in ClickUp, though it is on their feature requests and seems like something that may be added in the future.

For now, the ClickUp for Freelancers guide recommends using their Clockify or Timely integrations, both of which can track time from Clickup then can create invoices from the time tracked. Another option would be to include all of the info for an invoice in a new Clickup task, have an app integration tool like Zapier watch for new tasks with a filter to only watch for the ones you want to make invoices from, and then connect that to an invoicing app like Harvest, QuickBooks, or Wave to make your invoices.