This is because the Authorized redirect URI's are not entered correctly.
Ensure the following:
  1. Under Restrictions, enter the Authorized JavaScript origins for the domain of your school's Blackbaud K12 Core (ex.,
  2. Under Authorized redirect URIs, the address entered identifies the route where Google will send responses (ex.,
NOTE: It is recommended you include two versions: AND
For the full article and Steps, reference How do I set up Google Apps SSO API?