Sims 4 Won't Open In Steam

@RetroManVHSFan  If you've enabled family sharing in Steam, please disable it again.  It can block Sims 4 from starting.

Otherwise, please try playing in a clean user folder.  Move the entire Sims 4 folder out of Documents > Electronic Arts and onto your desktop, and when you launch the game, if it launches, a clean folder will spawn with no content.  (Your saves and other content will be intact in the folder you moved but temporarily not read by the game.)  Don't add anything to the new folder yet; just let me know whether you can get far enough to start a new save and play in live mode for a bit.

If that doesn't help, please run a dxdiag and attach it to a post.

https://help.ea.com/en-us/help/pc/how-to-gather-dxdiag-information/

@puzzlezaddict this didn't do anything. it still had the same problem as before. I don't know what happened, but after hours of trying to get it to work, The Sims 4 finally launched and I played for an hour or so. Today, I just tried playing and keep getting this error..."This file does not have an app associated with it for performing this action. Please install an app or, if one is already installed, create an association in the Default Appls Settings page."

this happened first. then I remembered your reply here so I tried that. I moved the Sims file onto my desktop and clicked the desktop link for The Sims. And again, it gave me this same message.

Here's the file....

@RetroManVHSFan  Try launching the game through Steam instead.  The desktop shortcut may be broken, but that shouldn't affect the game itself.

Are you getting exactly this error when double-clicking the desktop shortcut?  That would be odd unless you changed its file extension.  The message means what it says: that Windows doesn't know what program to use to open this type of file (designated by file extension, like .txt or .zip).  This only happens when you've never explicitly chosen a program for this extension AND Windows doesn't already "know" what to do with it.  But Windows does know what to do with a desktop shortcut that points to an executable, without any help.

Point is, you can sidestep this particular error by launching through Steam, and if the game doesn't launch through Steam, it's not because of the desktop shortcut.

Separately, what antivirus do you use?  Does it help to set exceptions for TS4_x64, TS4_Launcher_x64, and TS4_DX9_x64?  You may need to repair the game in Steam ("Verify integrity of the game files" or similar) afterwards.