Error: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'comments' cannot be null

I have already tried checking for related post but it doesn't seem to help me fix my problem at hand. I am trying to create an page for appointments where data are saved in the database. however, these two error message kept appearing on my window:

Notice: Undefined index: comments in C:\xampp\htdocs\db\connect2.php on line 29 Error: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'comments' cannot be null

here is the code php

<form name="appointments" action="" method="POST">
<label>Name (First/Last)</label> <br>
<input type="text" name="name" id="name" required="required" placeholder="Please Enter Name"> <br>
<label>Email Address</label> <br>
<input type="email" name="email" id="email" required="required" placeholder="youremail@yahoo.com"> <br>
<label>Phone Number</label> <br>
<input type="text" name="contactno" id="contactno" required="required" placeholder="9221234567"> <br>
<label>Nature of Appointment</label> <br>
<select name="service" id="service">
<option value="other">Other</option>
<option value="consultation">Consultation</option>
<option value="Surgery">Surgery</option>
</select> <br>
</div>
<label>Preferred Appointment Date</label> <br>
<input type="date" name="prefDate" id="prefDate"> <br>
<label>Comments</label> <br>
<textarea rows="12" cols="40" name="comments" form="usrform" placeholder="Your comments here..."></textarea> <br>
</div>
<input type="submit" class="btnRegister" name = "schedule" value="Send Your Request">
</form>

Here is my connect2.php

<?php
    //Local Server Information
    $server = "127.0.0.1";
    $username = "root";
    $password = "";
    $db = "myDB";

    $name = "";
    $email = "";
    $contactno = "";
    $service = "";
    $prefDate = "";
    if (isset($_POST['comments']) && !empty($_POST['comments'])) {
        $comments = $_POST['comments'];
    } else {
        $comments = "";
    }
    //Check if connection was successful
    try {
        $con = new PDO("mysql:host=$server;dbname=$db","$username","$password");
        $con->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
        if(isset($_POST['schedule']) )
        {
            $name = $_POST['name'];
            $email = $_POST['email'];
            $contactno = $_POST['contactno'];
            $service = $_POST['service'];
            $prefDate = $_POST['prefDate'];
            $comments = $_POST['comments'];


            $insert = $con->prepare("INSERT INTO appointments(name, email,contactno, service, prefDate, comments) values(:name, :email, :contactno, :service, :prefDate, :comments)");

            $insert->bindParam(':name', $name);
            $insert->bindParam(':email', $email);
            $insert->bindParam(':contactno', $contactno);
            $insert->bindParam(':service', $service);
            $insert->bindParam(':prefDate', $prefDate);
            $insert->bindParam(':comments', $comments);

            $insert->execute();
        }
    } catch(PDOException $e) {
        //die("Oops! Something went wrong with your database.");
        echo "Error: ". $e->getMessage();
    }
?>

This is the line where the error says the problem was.

$comments = $_POST['comments'];

I already tried hard coding it like $comments="you comment"; It went through with no errors and the data appears on the database. However, when I use the code above, error appears. Anyone help me please. Did i miss something? NOt sure what went wrong since the other line seems to work.

thanks