We usually see this error when the teacher and students are not using the same curriculum. Please make sure the student is linked to your account and that they are logging in so that they are served the right curriculum.
How do I know what curriculum I'm using?:
To check this on the computer: Look for the flag in the upper right-hand corner.
To check this on mobile devices:
Click on the purple menu from the right-hand side, and then select Settings.
From the drop-down menu, select Settings. Look for the flag icon.