You can't use the free tick option at the time of PR creation. Because PR is an internal document.
Its only about for your requirement and its totally with your organization.
The information will not goes to any vendor, customer etc.
In normal business process, we can't decide that the material will be delivered as free of cost at the time of PR creation. So the option is not available for PR creation.
You can see PR doesn't contain the price price which is calculate at the time of material delivery.
PR contain just material price.