403 message when pressing Process Payment button

I get the following message when I click the "Process Payment" button:

403 - Access Forbidden!You may not be able to visit this page because of:

1.an out-of-date bookmark/favourite
2.a search engine that has an out-of-date listing for this site
3.a mistyped address
4.you have no access to this page
5.The requested resource was not found.
6.An error has occurred while processing your request.
Please try one of the following pages:


•Home Page

If difficulties persist, please contact the System Administrator of this site.

Any ideas???????

(SOLVED)

Joomla admin GLOBAL CONFIGURATIONS had "Allow User Registrations" set to "no". It must be set to "yes"

All good