The file must be at a publicly accessible url. If the url requires you to be signed in or if it is a “viewer” url, the file cannot be uploaded using code.
Google docs also are not natively stored in a format that can be uploaded at files anywhere. They are cloud based documents.
You can convert you Google doc to a pdf file and then set the permissions of the pdf file so that it is publicly available. Then you can upload that url to an Airtable attachment field.
