When you click on the actual error on the google checkout integration tab, what's the full error description? Are you sure your sandbox is pointing to the right responsehandler.php? Are you using your sandbox merchant id (which is different than production)? Did you set the url callback for sandbox to http or https and if https do you have one of Googlecheckout supported SSL installed?
Have you configured shipping accurately in both Zen Cart admin and googlecheckout? When you test your respondhandler using the test tool did you get any error?
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