What have you modified in the site?
MY next step would be to
1. backup my work to date.
2. Drop an sql file of the database
3. Delete the database
4. Delete the web site.
5. I'd upload a fresh copy of PS3
6. Run the install program
7. When you reach the admin stage input the paypal express details
8. Create and item costing $1
9. register an email address and buy that item.
You have then eliminated all your work and are left with a problem either with paypal or the WA code. If it still does not work then there is code on the forum you can add to pages for fault finding but it is essential that you work with a clean installation.
Let us know how you get on
I haven't changed anything on the site except added the information to build it and the required PayPal and UPS information. I tried adding the code above to the "pp_checkout_failure.php" page but it showed nothing except the same error message. I copied and pasted it in both the beginning and the end of the code presently on the page.