Hi Manisha,
Invalid grant error usually indicates that there are permission issues due to the Secret Key (and all tokens that were generated using that key) being invalidated. This could happen when the Gmail password is changed or when 2FA is enabled.
To resolve this you’ll need to regenerate tokens by going to the console.developers.google.com > Credentials page. Click on the Edit screen for your credentials and Reset Secret after that. Here’s the button you need to click: https://take.ms/2EQIn
Alternatively, you can go through all the steps in our setup tutorial again.
I hope this helps!