I contacted PayPal, and spoke directly with Merchant Tech Support.
I was using a Standard PayPal Account and not an account that had been converted to a Website Payments Pro Account. (in a sandbox)
I have converted to a Website Payments Pro Account and rebuilt all my cart and checkout pages.
But I am still getting failed transactions when I checkout/process the payment on my website. According to merchant tech support at paypal, everything is set up properly on the paypal side.
checkout_failure.php displays this error message:
This transaction cannot be processed. Transaction Not Completed.
Not sure what to do next. Any ideas?