Make sure the web pages where you are placing your Online Express donation form are SSL-enabled. Please refer to the following for more information on SSL: Security and Online Express

It is important to note that once the Online Express donation form has been placed on the SSL-enabled web page, this error may continue to appear in Online Express for up to three days. The Online Express code checks for non-SSL page loads over the past three days. This error message will expire and disappear from Online Express after that time period has passed.