In the case your scholar is not signed in to their greatheartsonline.org account and your device is set up as a child device with Family Link enabled, you may need to explicitly add websites to the list of supported sites.
While this is outside the boundaries of our support, we can refer you to Google's instructions at
https://support.google.com/families/answer/7087030#zippy=%2Cchange-website-permission-settings%2Cblock-or-allow-certain-sites.
For the purposes of participating in Great Hearts Online, please sign in to the device using the scholar's greatheartsonline.org account. See: Setup for Personal Chromebook Device for GHO Use for Scholars