When clicking the Sign In With Google button in ClassLink, Google is signing in with an active Google account, a personal account that does not have access to ClassLink.
Make sure to log out of Google. You may go to https://accounts.google.com/Logout to log out of Google.
Then go back to your ClassLink login page to log back in. This time, you will be prompted to input your school-issued Google username and password.
If you are on a personal device, you may also need to install the ClassLink extension. For more info on ClassLink browser extensions, please visit https://support.classlink.com/hc/en-us/articles/115006588748-Training-Materials
Finally, here is information about using the ClassLink app on a mobile device: Android, iPad or iPhone. https://support.classlink.com/hc/en-us/articles/115006403607-Logging-In-Mobile-