It looks like there's something else called Afisho_rankimin in your DB so the function is not being created. Try calling your function something else. E.g.
CREATE FUNCTION dbo.Afisho_rankimin1(@emri_rest int)
RETURNS int
AS
BEGIN
Declare @rankimi int
Select @rankimi=dbo.RESTORANTET.Rankimi
From RESTORANTET
Where dbo.RESTORANTET.ID_Rest=@emri_rest
RETURN @rankimi
END
GO
Note that you need to call this only once, not every time you call the function. After that try calling
SELECT dbo.Afisho_rankimin1(5) AS Rankimi