not able to add google calendar in elementary calendar Sign-in attempt was blocked [email protected] Someone just used your password to try to sign in to your account from a non-Google app. Google blocked them, but you should check what happened. Review your account activity to make sure that no one else has access.
The right answer depends on if you use 2-factor authentication (2FA) or not.
You have to create a per-app password in your Google account here. Afterwords you will be able to login using that single-use password with your account.
In this case I think you have to tell Google to trust less-secure apps in your Google account settings (it just means they use standard authentication methods instead of Google-made methods). I cannot confirm this works as I no longer have any account without 2FA.
Hope it helps :)