You might want to check a couple of calibre threads:
DRM workaround: Kindle for Android and
Dealing with Kindle for PC/Mac 1.19 and KFX in calibre.
Did you install the serial number for Kindle for which you downloaded the file using Download and Transfer using USB in the DeDRM's configuration?
And that's about as far as we can go on Mobileread.
You can check the discussions on
noDRM's github page. If you are not currently using DeDRM 10.0.9, you can download it from
this link.