This is due to a student's Classlink account not being linked to their school Google account. You can head to the Classlink login page by clicking the home button and still sign in manually using their username and password. From there, they can connect their account in the sign-in options underneath their profile settings.

This will vary per teacher, but generally the link will be available inside a teacher's Google Classroom either inside their stream or labeled on their "classwork" tab. Additionally, through Classlink you can reach the WebEx Portal app where students can see a list for any scheduled class sessions they're a part of.