[Sirtel]

You need to configure the deDRM plugin with the serial number of your Kindle.

And no need to download the books to the Kindle. Use the Download & Transfer via USB option in your Content & Devices to download the file to your computer.

AND, DRM is removed on import, not when converting. So after configuring the deDRM plugin, you'll need to delete the file from Calibre and import it again. DRM won't be removed from a file already in the library.