Google checkout not working getting Oops! page when button is clicked???

hi all i have installed google checkout for a customer of mine but when some one adds a product to there cart and clicks on the google checkout button it takes them to a google error page,

the page says

Oops!
We were unable to process your request.

could anyone let me know why this is????

also do you need to have a SSL Certificate installed on the site for google checkout to work??? think i seen some were that you do. Could this be the problem?? as the site doesn't have a SSL Certificate??,

please get back to me and let me know, thanks from Patrick

I get that error when I am signed in as the same account as the zencart account.
Sign out from Google then try again.

also do you need to have a SSL Certificate installed on the site for google checkout to work??? think i seen some were that you do. Could this be the problem?? as the site doesn't have a SSL Certificate??,
Google checkout in LIVE mode will not work without
installed SSLCert approved by Google..i.e.
http://checkout.google.com/support/sell/bin/answer.py?answer=57856