The error can happen because of 2 reasons.
1. You are trying to connect your google account with Tars on a Chatbot which you do not own.
To resolve this, just make sure that you are connecting your Google account on a chatbot which you own.
2. You have already connected your google account with Tars and now google is not issuing the refresh_token again.
To resolve this, do this:
Go to this page: https://myaccount.google.com/permissions .This page shows the list of 3rd party apps your Google account is connected to.
Find Tars Chatbot in this list. Click on it to expand and then Click on the Revoke Access button.
This will reset any previous connected account with Tars Chatbot and allow to connect your Google Account to Tars Chatbot afresh, with proper refresh_token in place.