Thank you JSMACK for your reply!
After a long and tiresome web search I solved the problem (not entirely though

).
I uninstall Houdini 18.5.351 (which happened to be Python 3 version) and reinstalled Houdini 18.5.351 (non Python version).
Now I have to handle another problem.
Here is what it look like Houdini launched with 4096 X 2160 monitor resolution at general UI scale 1 which I rarely use because the icons and menus become too small and barely readable but the renderman preset material palette looks normal.

The problem arises when I change general UI scale from 1 to 2, which is my usual preference setting, renderman material preset palette becomes weird.

It is jammed with separator which seems to become larger by 2 times and the content windows shrink so much.
Is this a bug or did I do something wrong? Is there any trick to solve this problem?
By the way, where can I find json file and how to verify if that is good?
By the way, where can I find json file?