I've read that mobidedrm prefers the 2.6.x branch of python instead of the 3.x
Try using that version of python (I can't confirm or deny that that is an issue). Also if your using a windows system the single quotes aren't required.
Lastly if your books have spaces in the file name you either need to rename them (what I'd recommend) or put them inside of quotes.
mobidedrm I_like_really_long_titles.prc New_File_Name.mobi" 'PID'
mobidedrm "I like really long titles.prc" "I like really long titles.mobi" 'PID'
====UPDATE====
CONFIRMED 2.6.x branch only
https://www.mobileread.com/forums/sho...t=34190&page=3