The above error is typically caused by the student using the wrong URL to log in to your Learning Management System. LockDown Browser for Chromebook requires that you use the exact same URL that is specified in your institution's Respondus server profile.
To find out what that URL is, please contact your institution's help desk.
It's also essential that when logging in via that URL, a redirect doesn't occur to a different URL. If so, the error will persist. If you see the URL change to something different in the Chrome browser's URL window, your institution's network administrators will need to look into the matter to see if they can prevent the redirect from occurring. Support for vanity URLs may be added at a future time.