The only choice you have to get it on your computer is the Kindle for PC.
Dang thing acts up all the time.
First if you have a Firewall, disable it then try to download the book on the Kindle for PC. Your Firewall might be blocking it.
Next failing that
Uninstall and then remove all left over parts of Kindle for PC left on the hard drive.
Before you do this, MOVE all your Kindle books from that folder to a safe place. I would say simply pull up the folder and relabel the name for now.
Download and do a clean install after removing all the files related to Kindle for PC.
Try the download again.
IF that works move your books back into the folder labeled for My Kindle and you are good to go.
If that doesn't work, I really hate to do this because no human should have to deal with Amazon's messed up service, but you'll have to go to Amazon support. Just tell them you can't download to Kindle for PC. Don't tell them about the ereader and Calibre.