It seems that the function ‘json_encode’ is not available in your PHP configuration. Either your hosting provider has disabled it for some reason or the PHP version on your server is lower than 5.2.

Most likely your PHP version is 5.1 which does not support json_encode by default. PHP 5.1 is not supported by opencart and if this is the case, you should ask your hosting provider for upgrade to PHP 5.2.

Regards, ThemeBurn team