I ran into the same problem a couple of days ago, exactly as described at
https://unix.stackexchange.com/question ... ssword-setA couple of answerers suggested using sudo or kdesu but I didn't try that after reading this post:
Don't run partitionmanager like this. Things might break if you use sudo (note, I am the maintainer of KDE Partition Manager). – Andrius Štikonas Aug 27 '18 at 11:47
I just wanted to do some partitioning, not track down bugs, so I installed Gparted and it worked straight away. Next time I'm on that computer I will apply the above solution as it's a brand new Buster/KDE Plasma install and I'd like to keep it using KDE programs where possible and ditch Gparted.
Thanks Head_on_a_Stick! It certainly is curious why it's not set up for pkexec and the bug appears to have existed for some time.