If you want to convert your mouse position (in pixels) into coordinates in your game world then it's
mapPixelToCoords.
As you can see in the documentation it is not a static function, you have to call that method on an instance of RenderTarget.
You have to pass a Vector2i (it will be passed as a reference, don't try nonsensical things like
Position2&) to that function, and the sf::View (not a RenderWindow) you want to convert your position in (you may (or will, eventually) have multiple sf::View). If you don't pass an sf::View then it will use the current view of your window, which may be ok.
Note that it returns a Vector2f (floats), not a Vector2i (ints), so if you store the result in an sf::Vector2i you may lose precision.
There is an example of what you're trying to do at the end of the
view tutorial.