It sounds like you are testing our Buy Now button code on your site. You cannot test Buy Now buttons by setting the price to 0.00 because Buy Now buttons take a buyer directly to checkout for payment, and PayPal cannot process a "payment" for $0.00.
If you want to test Buy Now buttons, you will need to set the price to some actual amount, even if only 0.01, then borrow someone else's PayPal account or have them click your button themselves to make an actual test payment.