https://help.tallysolutions.com/docs/faqs/all_releases/8659.html

Error: Authetication Failed(Gmail)

Cause

This error is displayed when you have not enabled the option to allow external applications to access your Gmail account, under Sign in & security in Gmail .

Solution

To resolve this error

1. Open your Gmail account.

2.

3.

4.

5.

Now, you can access your Gmail account.