If you want to upload an attachment using an automation, scripting, or third party integration, the attachment must be hosted at a publicly accessible url.
Airtable cannot upload files from viewer URLs that show the file in a viewer (versus the raw file) or URLs that require a user to be logged.
Accessing attachments shortly after upload
When you upload an attachment, the url starts out as a temporary url, for a fraction of a second (or sometimes a little longer), so it is best to wait a few seconds after uploading the attachment before attempting to access the file.
In scripting, I tend to wait a few seconds and then check for a thumbnail url. If the there is a thumbnail url, the attachment is good to go. If there is no thumbnail (for a file type that can get a thumbnail), I wait a few more seconds. If there still isn’t a thumbnail, I either fail and warn the user or I try again with a cache-busting variation on the source url, depending on the use case.