This means your host is likely blocking loopback requests. You can install the Health Check & Troubleshooting plugin to verify this. iThemes Security doesn't use loopback requests, but it makes the check to identify the loopback IP so Trusted Devices doesn't block it. If loopbacks aren't working on the site iThemes Security will still work fine because nothing will be able to successfully make a loopback request. But you should speak with your host about getting this fixed as it could cause other troubles.