Can't log in to my account in launcher or game but can on the Sims 3 Store

abistasiasims  You shouldn't need to login to the launcher to install your Store content.  Are you being asked to do so?  In that case, do you own multiple copies of Sims 3?  For example, I've registered a disc copy and one from Steam.

Please also test in a new admin Windows account.  Make it local, as in, don't link it to your Microsoft account or any email address.  You'll be able to test whatever you want without reinstalling anything.

The "serious error" is usually OneDrive-related.  Please see the first post of this thread if you run into it again going forward:

[Current Issue] Sims 3 "serious error" when loading save | EA Forums - 8322938

Playing the game in a local admin account fixed everything. Does this mean I will have to log into a different windows account when I want to play TS3 now?

As for your first question, when I am on the store's website and hit download (using my main Windows account), it opens the launcher on the login popup and if I exit out my item isn't there in my launcher for me to install. And my only copy of the Sims 3 is from a disc. 

abistasiasims  This means that the problem is confined to your main Windows account, which is good, that means your EA account is fine and so is your computer overall.  The question is what data or what setting is the problem.

In your main account, please clear Edge's Internet Explorer-mode data.  Even if you never use Edge, let alone IE mode, the launcher does for some purposes.

https://support.microsoft.com/en-us/microsoft-edge/view-and-delete-browser-history-in-microsoft-edge-00cf7943-a9e1-975a-a33d-ac10ce454ca4

If that doesn't help, clear all Edge data, in case something else is lurking there that isn't accounted for by IE mode alone.

From what I can see, I don't have the option to specifically delete Internet Explorer data, so I did the "Delete browsing data" one and it didn't fix it. Is that what you mean by clearing all Edge data?

abistasiasims  Towards the end of the article, there are instructions for specifically clearing out Internet Explorer-mode data.  I don't know whether what you did already would have deleted this data too, so please check and clear whatever is still present.

I saw that originally but that is what I don't have the option to do. The article has a note with it that says "This is only available if your organization has turned on Internet Explorer mode." so it's probably the case that I don't have that turned on. 

abistasiasims  It should be there anyway on a personal computer; the wording is more for companies with IT departments.

At any rate, try using a new admin Windows account, as a test.  Make it local, as in, don't link it to your Microsoft account or any email address.  Your saves and other user data won't be available, but that can be addressed later if necessary.  For now, this is just a test.

The local admin account worked to let me launch the game and be logged in.

abistasiasims  Great, so at least the problem is contained to your main account.  There are a few players with the same issue, and I'm still trying to figure out where exactly this data is stored, but in the interim, you can play in the new account if you want.  For any saves or other user data you want to transfer, just drop it in C:\Users\Public, which is accessible from any account.  Grab it from there and put it in the Sims 3 folder in your new account, or just transfer over the entire Sims 3 folder.

If you want to look around a bit, go back to your main account, download Piriform's CCleaner (the free version is fine), and search for files with sims 3 and sims3 (without the space).  You'd only want to remove files you didn't recognize and that were in your main account, as in, something that the new account wouldn't be using.  I don't know exactly what you'll find, but if this isn't a task you'd like to deal with, let me know and I'll test it myself later.  You can also simply document what you find, not deleting it, and list it here for us to go over.

I just tried to use the CCleaner to search, however, I don't think I did it right. I used the duplicate files finder and then scrolled trying to find anything with those in the name. I didn't find anything, but if I did this wrong, let me know and I can do it another way!