Notice how “Sign In” is adjacent to the Online ID field. But the Account Location dropdown is below and separate. Yet it is required!
This flow makes the user enter their online ID and hit Sign In, but ignore the Account Location field because it does not appear to be required given its positioning.
And then people get this error…womp womp.
The ordering, labeling, and existence of your fields and buttons should make sense. No wonder they need a help/options link in the very same box.