Getting "This app isn’t verified" error while connecting my Google Account with Google Calendar addon.

When you get this error, please do not panic and work patiently as it is only for a short time.
Google Account Verification process is under verification mode and under review. When it is verified, it will show you the verified consent box.
Currently, it will show you a message about the unverified app. You can proceed with unsafe mode.

Take a look at the following image for the option