PDF does not contain fillable fields. for external storage

If you cannot access the file from the command line, pdftk will not be able to see it. The error message could probably be improved to indicate that it cannot see the file at all instead of being able to see the file but it has no fields.

Templates can be accessed from the template listing page with external URL.
Can you show up the steps or commands to access it from command line?
To confirm is this module use an absolute URL or a relative for the template to access?

That is the problem. pdftk needs to be able to access the PDF from the file system. If it is remote, it is not on the file system. To make it work, the PDF would need to be copied to a temporary location on the file system. That would be a feature request.

Perhaps FillPDF should be able to detect that a storage configuration does not store files where they can be accessed from the file system and prevent those from being used.

Can we use another service(FillPDF LocalServer ) to avoid this problem ?

There is a good chance that that would fix it.

I've configured the FillPDF LocalServer and is working really well for the template files served from Azure Blob Storage.