"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.
Here are the steps on how to fix the Google blocking app error:
Login to your Google Admin account.
Navigate to the Users tab and double-check that the users you are trying to connect to Instantly are listed.
Go to "Security" > "Access and data control" > "API Control" > Manage third-party app access.
Make sure the oAuth has been enabled for all the users on this workspace.
Login to your Instantly account from Incognito mode.
Try to add the Google account again.
Note: If oAuth isn't set in this workspace, you may read this article here.
To watch the full walk-through video, you may click here.