Sims stuck in windowed mode

@BeeamerrBoyy  What resolution is the game running at, and what is the resolution of your screen?  For the former, open the graphics options, and you'll see it listed near the top.  For your screen, the resolution will be under Windows Settings > System > Display.

The screen resolution is 2560 in my settings and in the game options.

@BeeamerrBoyy  That's half the resolution.  What's the other half?

The other half is 1080

@BeeamerrBoyy  Please run a dxdiag and attach it to a post.

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

Here it is.

@BeeamerrBoyy  It may not be related to this issue, but please clear at least another 15 GB on your C drive.  You currently only have 6 GB free, which is barely enough for Windows to maneuver properly even without adding Sims 4 to the mix.  This is important both for gaming and for the overall health of your computer.  While your C drive is somewhat small, your D drive is more than large enough accommodate any overflow.

Once you've done that, restart your computer, then download the newest driver for your graphics card from Nvidia:

https://www.nvidia.com/Download/driverResults.aspx/203214/en-us/

Run the installer as an admin: right-click the download and select "Run as administrator."  Restart your computer after installing the driver and before doing anything else.

Before you try to play, go into Documents > Electronic Arts > The Sims 4 and move the options.ini file to the desktop.  This will reset all your in-game options.  Load the game and let me know what resolution you see in the graphics options.  If you still see black bars on either side of the game, switch to windowed mode, on the same graphics screen, quit the game, restart, and check again.

I tried everything you told me, when it loaded the game it came up as 1,920x1,080. When I loaded it up again it said the same thing. I checked what my monitors settings are and it says 2,560x1,080. Just to let you know I moved my options to the desktop and tried it again in windows mode and downloaded the application and everything’s still the same. Also it’s only in windows mode in the world menu, in everything else it’s fullscreen. 

@BeeamerrBoyy  Does the game give you the option to switch its resolution to 2560x1080?  Please try it if so.

It's possible that the worlds menu is a special case: not supported properly for widescreen, or not able to stretch properly to fill the screen when the resolution doesn't match.  That would take some more testing that you may or may not want to do, but the important thing is that (now at least) the rest of your game looks like it should.

Yes it does give me the option to switch and whenever I do it just stays windowed, and yes I would love to do more testing.