KPST is what's known as a stuff-pack. You have EPs which are expansion packs and stuff packs which contain...well...lots of stuff to beautify your sims' homes or add vehicles or stuff like that (building stuff). Katy Perry's Sweet Treats was a limited time stuff pack that was done in collaboration with Katy Perry and it contains a whole bunch of sugary sweet decorations to litter your lot with.
The game tells you what packs you don't own. And of course, now that EA no longer has that collaboration agreement they can no longer sell the stuff pack through EA App/Origin. It seems the only place where you can get KPST is through either Amazon or eBay.
Unfortunately, I don't have Stiff As a Board, Light as a Feather...
Another item you might find of use is Lord Vladimir's Conjuring Cauldron. It's a lot of fun if you have supernatural sims...even non-Supernatural sims can use it. Although don't use it too much at one go or your poor sim might end up like Haruo here.
https://i.imgur.com/r1g29M6.jpg
