In a previous question, When shopping for ebooks on Amazon how do I tell if they have DRM?, it is explained that DRM-free ebooks on Amazon are characterized by the "Product Details" item: "Simultaneous Device Usage: Unlimited".
I was a bit surprised though that all such books I looked at also had: "Lending: Not Enabled", which seems a bit contradictory. But this is a side remark.
My problem is that I do not know how to download these (supposedly DRM-free) ebooks, once acquired.
I did go to my Amazon page "Manage Your Content and Devices", where all my Amazon books are listed, one book on each line.
There, it seem that to download a book. I must click it in the Action column (the second square of the line), and then select an action, which apparently should be "Download & transfer via USB".
But when I do, I get an error:
Download & transfer via USB
Select a device from the dropdown
John - Android Tablet
John's Kindle Cloud Reader
You have selected content that is not compatible with any of your registered
devices. Please remove incompatible content from selection for eligible devices
Transfer Tip: After downloading, use your USB cable to connect your computer and
Kindle. Your Kindle will appear as a drive on your computer. Copy your downloaded
file from your computer to your Kindle's documents folder.
All I want is to get the MOBI file in my computer, as I do with other ebook sellers for DRM-free books.
I am running Linux, if that can be any help. But I do not own a Kindle, and my only devices are my PC (running Calibre on Linux) and an Android pad used by another person.