If you are seeing this when trying to access Google Classroom on an Android phone or tablet, you will need to install an app called 'Device Policy' (made by google) from the Play Store. Follow the steps in that app.
You also need to have a screen lock on. Perhaps just '0000' for younger children or a pattern.
It will then create a 'work profile' and download a second version of google classroom with a picture of a briefcase/suitcase next to it. You must then use this version of Classroom.