Just wanted to share my results after spending last evening getting PM loaded onto my new Nexus 5X I snagged off ebay.

I was running into the issue that I'm seeing happening to a lot of people on this forum, and I eventually got it resolved and Plasma "working" (as much as it is in this state) finally on my device.

Most of the workarounds were not exactly working for me, such as flashing the stock 6.1 image in the flash-vendor script or unmounting the cache mount partway through the install when it grabs sudo permission.

What ended up steering me in the right direction was this tutorial here. Basically, going directly to google and getting the oldest factory image for the 5X is what put it in a state where it would flash properly.

TL;DR:
1) Grab this this archive directly from google's developer website (I'd like to guarantee it's safe, but, well, you know, google).

2) Unzip that, then when it finishes, put your phone into recovery mode (Volume Down + Power).

3) Open the newly unzipped bullhead-mmb29k directory, and run the flash-all.sh script
WAIT FOR IT TO FINISH - the first time I ran it, it looked like it was complete, and I interrupted it before it was truly finished. Be patient!

4) Once that process has finished and you've completely booted into stock Android 6.0.1, shut the phone off and put it into recovery mode once more.

5) Now, follow the Plasma Mobile instructions from the beginning.
MAKE SURE YOU FORMAT THE CACHE AND USERDATA BEFORE RUNNING pm-flash AGAIN (Again, this is another thing I missed and took extra time solving)

6) After you have formatted the cache and userdata, run the pm-flash script once more. I would go so far as to re-clone from git again just in case you modified anything (I did).
Let that process finish, and you should get your "OH IT'S GORGEOUS!"

Hopefully this process resolves the issue for you as it did for me. I'm a big KDE fan and I really hope this ends up maturing into a really competitive mobile platform in the future!