This error will occasionally get thrown by Google accounts for a variety of reasons. In most cases this error simply will get thrown when Google wants to verify that the owner of this inbox is the one attempting to connect this inbox to the platform. To resolve this issue, please follow these steps below.
1. Open up the 'Add Inbox' page under your dashboard and enter in your connection information but DON'T try to connect yet
2. Navigate to this link under the Google account https://accounts.google.com/b/0/DisplayUnlockCaptcha and click "Continue"
3. Hit "save" on the add inbox page
This essentially gives Google a heads up that you are attempting to make a new connection to your Google inbox.
Note: For users who are repeatedly experiencing this issue and having consistent connection issues with the platform, we recommend getting setup with 2 factor authentication under Google and creating an app password to connect this inbox to the platform. You can find a guide on how to get setup with this here.