Not only your Stored Procedure has a syntax Errors but it also has a logical mistake. You are inserting a record and then checking if that record is present or not. [Edited - For spelling correction.]
Change your Stored Procedure as below.
set ANSI_NULLS ON
set QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [dbo].[insertUser] (
@UserName varchar(50),
@Password varchar(50),
@FirstName varchar(50),
@LastName varchar(50),
@Email varchar(150),
@Mobile varchar(50),
@Result varchar(100) output )
AS
BEGIN
Declare @UserCount int;
SELECT @UserCount = COUNT(*) FROM users WHERE UserName = @UserName;
IF(@UserCount > 0)
begin
Set @Result = 'User already exists';
end
ELSE
begin
INSERT INTO users(UserName,Password,FirstName,LastName,Email,MobileNo)VALUES (@UserName,@Password,@FirstName,@LastName,@Email,@Mobile);
Set @Result = 'User successfully added';
end
END