"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.
Login your Instantly account from Incognito mode.
Try to add the Google account again.
If the incognito doesn't help, please follow these steps and double-check that oAuth has been enabled for all your domains/workspaces.
Login your Google Admin account.
Navigate to Users tab and double-check that the users you are trying to connect to Instantly are listed.
Go to API Controls and click 'Manage third party-app access.'
Make sure the oAuth has been enabled for all the users on this workspace.
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.