Full Error Message: 
“Error while creating new document. Error getting access token. Error: invalid_grant. Error description: Bad Request
  1. Go to DocGen Configuration tab to reauthorize app with your google drive following steps in this article.
  2. If you have done above step and still see this error, the app has already been authorized and you do not need to do this again. If you get the error when generating document after doing authorization, then it may be an issue with Force authorization feature under one user and needing to remove other authorizations. You can revoke each users authorization and then authorize under just the user who is forcing authorization.
  3. If did above steps and still seeing error message and not able to generate document, please install latest package from AppExchange to have latest app updates and features.