Be default invoices can only pull data from approved billable timesheet hours. This setting allows invoices to pull data from unapproved billable timesheet hours as well. Screenshot below shows the user setting on, which allows the user to select approved and unapproved billable hours.
Checking the "Allow invoices to be created using unapproved time" checkbox will then turn on the setting above which allows invoices to select unapproved time. See screenshot below which is located in the company settings tab.