When someone clicks on "Register My Account" it doesn't go

When someone clicks on "Register My Account" it doesn't go to payment screen (PayPal Express). Instead it goes right back to the membership signup page

Note:
- Membership levels have been set up.
- Gateway has been set up
I started with the free version. I set up the membership levels and tested with PayPal Exress in sandbox mode and everything worked fine.
I upgraded to the full version and now the sign-up/payment no longer works. As stated above, when someone fills out the form, and clicks on Register My Account, it just reloads the sign-up page.

Please advise ASAP.