If you are accessing Classlink from a device that isn't a district Chromebook, you will need to add your profile and sync your Google account. Here is what you do:
To access ClassLink, a user needs to be signed into the Chrome browser with Syncing.
Here's how to turn syncing on if it's off or paused:
Click on profile pic or initial in the upper right of the Chrome browser and make sure you are logged in with your school email. If it is a different gmail account, you will need to add a new profile.
Click on blue prompt "turn on sync"
Link Data
Once the Data is linked, you should be able to log into Classlink.