@MeganFord1213 If you don't know what mods are, you definitely haven't installed any. Custom content wouldn't cause an unknown error, either.
The good news is, your graphics card is in fact supported, so you can skip the most complicated fix for the unknown error. That leaves two steps. But first, run the Super Patch (digital download version) to patch the game to 1.67, if you haven't already. The launcher doesn't always do a thorough job of patching on its own, and the Super Patch won't hurt.
https://bluebellflora.com/sims-3-manual-super-patch-for-mac-1-67/
The first fix for the unknown error is to create an alias for each pack that doesn't already have one in Applications, alongside the alias for the base game. For you, those packs are probably Ambitions, Late Night, and Outdoor Living. Creating an alias is easy enough:
https://bluebellflora.com/sims-3-packs-not-showing-as-installed/
The next step is a bit more complicated: You need to copy files from each of those same three packs into the folder for Into the Future, which is your newest pack by release date (since it's the last one released). This video shows exactly what to do:
https://answers.ea.com/t5/The-Sims-Mac/CURRENT-ISSUE-Sims-3-Unknown-Error-in-El-Capitan-Sierra-High/m-p/5725514#M5926
After you've done all this, your game should launch. If it doesn't, the other workaround is to bypass the launcher:
https://bluebellflora.com/sims-3-how-to-bypass-the-launcher-on-a-mac/
I will say though that I don't think your computer will run TS3 very well, especially with Pets and Seasons in the picture. You have a dedicated graphics card, which is good, but it's old and was never one of the strongest models. Your 4 GB RAM will need to be shared between the game and your operating system, and High Sierra will claim a lot of memory for itself. But even if it didn't, TS3 in mac OS can only use 2 GB RAM at a time before crashing. Running the most demanding packs (Pets, Seasons, then Into the Future and Island Paradise) may put you close to the RAM limit as soon as you load a new game. Purging RAM will help a bit, but it's not really a solution.
https://bluebellflora.com/resetting-ram-during-gameplay/
Anyway, I hope you are able to play, at least a bit. Turning down your graphics options will help, as will disabling memories and the hidden object interactive loading screens. After the first time you launch, you should only play while your computer is disconnected from the internet; otherwise, EA servers will take up some of your computer's resources. And, of course, don't have anything else running while you play.