In order to invoice a customer, that customer must have a billing profile set up. To do this, navigate to the customer record, and to the deliver tab. You will see a sub tab there called billing profile.
Select 'New' and add the appropriate information in the billing profile. You have many options here to fill out to ensure that the right person is receiving the invoice and the right terms are applied. You also have the ability to choose an invoice template here as well.
If the customer does not have a billing profile set up, you will be unable to invoice them.