If you installed Circle Go on iOS prior to 9/1/2018, it’s possible that you will see a certificate error "NOT VERIFIED" when opening iOS Settings >> General >> Device Management after 9/9/2018. This might also be phrased as the "certificate expired".
Everything will still work for Circle Go on your device, but this alert may alarm you if you see it.
To resolve this, you'll just need to uninstall the MyCircle app, and then reinstall it. This will install a new certificate on your device and keep the error from reappearing.