"This app is blocked" error message usually indicates that oAuth has not been configured correctly for the workspace associated with that account. No worries! We're here to help you resolve it.
You can follow the steps below to fix the error:
Log into Instantly from Incognito mode. Try to add the Google account again.
If Incognito doesn't help, please follow these steps and double-check that oAuth has been enabled for all your domains/workspaces:
Kindly login to your Google admin account
Go to the "Users" tab and double-check that the users you are trying to connect to Instantly are listed there:
Go to the API Controls - "Manage Third Party App Access"
Make sure the oAuth has been enabled for all the users on this workspace
If oAuth isn't set in this Workspace, this article could be helpful: