Error: The statement type is not supported. it must begin with CREATE or ALTER

Hi! I have a problem when i implement your codes..

the stored procedure seems not be working..

the error is unable to save the project because the statement type is not supported. it must begin with CREATE or ALTER

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE PROCEDURE spx_CheckUserAvailability
      @UserName VARCHAR(50)
AS
BEGIN
      SET NOCOUNT ON;
      IF NOT EXISTS
            (SELECT * FROM Staff
             WHERE UserName = @UserName
            )
            SELECT 'true'
      ELSE
            SELECT 'false'
END
GO

so, i decided to change it.. but i still have a problem... 

the error is invalid object name 

the readjusted code is

ALTER PROCEDURE [dbo].[CheckUserAvailability]
      @Username VARCHAR(50)
AS
BEGIN
	SET ANSI_NULLS ON;
	SET QUOTED_IDENTIFIER ON;
	 SET NOCOUNT ON;
      IF NOT EXISTS
            (SELECT * FROM Staff
             WHERE UserName = @UserName

            )
            SELECT 'true'
      ELSE
	  BEGIN
            SELECT 'false'
END
END