Just refreshed myChevrolet app and it's working fine. Make sure your internet is connected if on wi-fi and working on other sites.
Not sure I can help you there as I'm still in my free trial period, so I do have an active account. I do not plan on continuing the OnStar service, so hopefully that's not the issue.Can you at least confirm that you don't need a paid OnStar plan to access that part of the service?