Hey nyanwils,
Thanks for reporting this bug!
I tested your problem in a freshly generated pixelmon single player world and I was able to craft & place a pixelmon painting perfectly fine.
Same test results applied to a newly generated test server - world.
The pixelmon version I was using is 8.1.2 (Make sure you are using the latest reforge one, link:
https://pixelmonmod.com/mirror/core/5.1.2/)
As I couldn't reproduce your bug, I am gonna mark this ticket as "Not a bug", if you still can not place paintings even in new worlds, please create a new ticket and add more information to it (i.e. what exactly happens when you try to place a painting, extra mods et cetera)
Kind regards, NotKili