Expansion packs telling me to install the Sims 3

Hello, I have a legitimate copy of the sims 3 downloaded from steam and I am trying to add basically all of the expansion and stuff packs to it. I have tried a couple of different packs but they all tell me that I need to install a copy of the sims 3 before I can install them.

Any help? I did search for an answer already, so I'm really sorry if I missed it but I tried my best!

Its because expansion packs are rely on the main sims 3 game which you say is store bought so your problem is probably that. Unfortunately you will have to use a non store bought launcher like the FairLight (FLT) one which I am sure you can find in google.