I can't enter full screen mode, it says "Unable to enter fullscreen mode - Can't set the video mode"
I have windows 7 .-.
How do I fix this?
Thank you.

Is it windowed mode or fullscreen? If it's windowed, it's because the game window set to your native res plus borders would be too big to fit the monitor, which is pretty annoying.

Set it to a lower resolution, Alt+Enter to fullscreen, then change it to the one you want.