So here is the way to browse and buy from Plopsy via the phone and computer ;)
Phone
red.....click on the phone icon
yellow...click on the house button
blue...browse plopsy (I would advise to scroll down if its not at the top like mine, it could be at the bottom)
https://i.imgur.com/kAZ95c0.png
Computer
step 1
red...click on computer
yellow....click on more choices
https://i.imgur.com/F29zY2M.png
step 2
red...click on order
https://i.imgur.com/d8pOwkm.png
step 3
click on browse plopsy (sorry I forgot to circle it, but I did hightlight it in green by hovering the mouse over it ;)
https://i.imgur.com/yuWTXFd.png
I hope this helps you. If you don't have any of the above options, it could be a mod or cc conflicting. Try running your game vanilla (cut and paste your ea folder to your desktop then run the game as if for the first time, it will create its own new ea folder). If it works that way then delete the new ea folder and put yours back but try and remove mods one at a time to see which one is causing your error. I do hope you come right, it must be so frustrating to not be able to use the gameplay of a pack you paid for. Happy simming :)