Check that you have the latest update to PayPal installed from:
http://www.zen-cart.com/forum/showthread.php?t=56216

Then go to the Modules ... Payments ...

Remove PayPal IPN and/or PayPal Express (both if using both)

Install both PayPal and PayPal Express (even if only going to use one)

Remove PayPal IPN or PayPal Express (if you are only going to use one)

Configure PayPal IPN and/or PayPal Express (either both or just the one you are going to use)

NOTE: only have installed Payment Modules that you are actually using ...

Then, if you still cannot use this ... switch to Classic Template ... does it work now?