You need to use the actual angle() expression here. You've just typed this in the angle box:
[color=#00BF00:g7zn6tkv]Sprite.X, Sprite.Y, MouseX, MouseY
and you need to type this:
[color=#00BF00:g7zn6tkv]angle(Sprite.X, Sprite.Y, MouseX, MouseY)
the [color=#00BF00:g7zn6tkv]angle(x1, y1, x2, y2)
[/color:g7zn6tkv] thing is an expression that gives a number that represents the angle between the two points of Sprite.X, Sprite.Y and MouseX, MouseY. It's a math function that's figuring out the angle for you based on those four coordinates:
For more information on math expressions, check out the [url=https://sourceforge.net/apps/mediawiki/construct/index.php?title=System_Expressions#Math:g7zn6tkv]wiki[/url:g7zn6tkv].