The screen only fills like 75% of my screen. When I click the shortcut. it gives me four options "Play","options", "support" and "exit". I click play and it only fills 75% of my screen. went to options and unchecked "windowed mode", but nothing changes. I went back to settings and it was like I didn't even change anything. changed it again and clicked"okay" went back to settings and it was changed back to the old settings.

Usually pressing "ALT + Enter" forces a game from windowed to fullscreen. Did you try that?

tried it just now, but nothing

When I first installed - just a coupla days ago - the very first time I fired up the game, it was not fullscreen.
I could still see my Windows taskbar at the bottom.
After about ten or fifteen seconds, I clicked on the windowed game-screen (just a left mouse-button click) and it snapped fullscreen.
No problem since! Game immediately starts in fullscreen, all by itself...
And I seem to remember this from my original Skyrim install too.

If running 1920x1080 desktop, and game at same resolution as well, fullscreen is easy enough to accomplish.
But if running a reduced rez (not your monitor's native size) this scaling to fullscreen may not occur so seamlessly...?
Is this your problem, friend?
If it looks like it gets more complicated, this thread must be moved into PC Hardware Problems forum.
But hopefully not a big (or 'permanent') issue for you...

I figured it out. I had a mod that replaced the original fall out 4 launcher , but I restored it back to the original launcher in the past. I thought the mod wasn't active because the original launcher was back. It was still active even with the old launcher back on. I could only change the settings through the mod and I couldn't change the settings through the original fall out 4 launcher. I reactivated the mod and I was able to change the settings through the mod. Works fine now. Thanks for trying to help.

"thanks for trying to help" = "sorry I wasted your time"