You may see this error message when generating a document under a Salesforce user where one or more fields that are being merged in the document are unaccessible to those users which is why they are getting the error. We had to implement this feature to be in regulation of Salesforce security policy so you will need to give all app users proper access to the field(s). For example, the field cannot be hidden from the users generating the document and they need at least “read” field accessibility.
Edit Field Permission(s):
You can go to Setup > find related field under object > then open up security permissions or add permission for each SF user that does not have proper access to it.
If you are not sure which field is returning the error or that user does not have proper access to from template, you can remove most fields except a few from google doc template, test and see if error happens. And isolate the error and update permissions. Or you can create a new test doc action (clone the other one returning this error for limited users) and create a new google doc template, update sharing settings on google doc template and then copy/paste a few fields at a time under restricted user to see which fields returning the error. And then update the permissions of field or for that user.
Image of Error Message: