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

Error: Authentication 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. Select Accounts in Google Apps .

3. Click Sign-in & Security .

4. Ensure the 2-Step Verification is turned OFF .

5. Scroll down to enable the option Allow less secure apps .

Now, you can access your Gmail account.