How do I get rid of or fix this error?

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PHP

I have been trying to validate my register form using javascript but i figured by using the: alert('it works") just to see if the js is working of the form but i am getting this error

Notice: undefined index: in gender in c;

which has something to do with the php code. to my js is non functional until that error is fix, tried everything that I can think of and even went searching the net and tried all that I can see but to no avail

can someone help and also it is not inserting into the database, can something tell me what is wrong and why it is not inserting into the database

maybe a third eye will help me what this is not inserting into the database, I am getting all kinds of errors that I cannot see, it give me wrong result when i run it

when i mean wrong result, I mean when a user is registered it, a popup box will popup and tell u welcome to the webpage and then it should redirect the user to the login page an it is not doing that, it instead telling the users that the username already exist ( which I dont have within the code itself) and it either take u back to the index page or back to the register page which not supposed to do

What I have tried:

this is what i have

<pre><?php

		//$register=$_POST['register_user']; 
		if(isset($_POST['Register']))
		{
			
			//capture the variable from the form and store in php variables
			$title=$_POST['title'];
			$firstname=$_POST['firstname'];
			$lastname=$_POST['lastname'];
			$username=$_POST['username'];
			$email=$_POST['email'];
			if(isset($_POST['register']))
			{
				if(isset($_POST['gender'])) 
				{
					$gender = $_POST['gender'];
				}
			}
			else
			{
				$gender = "";
			}
			$address=$_POST['address'];
			$mypwd=$_POST['mypwd'];
			
			include'db_server.php';
			
			//Query the database
			$sql="SELECT * FROM members WHERE username='$username'";
			
			$result= mysqli_query($conn, $sql) or die ("ERROR:" .mysqli_error());
			
			$rowcount=mysqli_num_rows($result);
			
			//checking to see if username is already exist
			if($rowcount >= 1) 
			{
				echo "<script type=\"text/javascript\">
					  alert('Welcome!! Firstname Lastname, you are now a member of the Caribbean Nature Seekers Institute TT(CNSITT)');
					  window.location=\"../login_user.html\";
					   </script>";
					   
				 
			}
			else
			{
				//insert data into table
				
				$sql = "INSERT INTO members
				VALUES('$title', '$firstname','$lastname', '$username', '$email', '$gender', '$address', md5('$mypwd'))";
			
				if(mysqli_query($conn,$sql))
				{
					session_start();
					$_SESSION['user']=$username;					
					header("location:login_user.html");
					mysqli_close($conn);
						
				}
				else
				{
					echo "Error inserting values into database";
				}
				//end of line
				
			}
		}	
		
			
?>

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