New Password and Confirm Password Field do not match error

here when I'm trying to register myself a popup says localhost "New Password and Confirm Password Field do not match !!" even after entering the email, phone, name, password, and correct passwords, it is showing me the same error New Password and Confirm Password Field do not match error

I am new to it and I can't understand what is going on. Please help me! How can I solve this?

<?php
    //error_reporting(0);
    if(isset($_POST['signup']))
    {
        $fname=$_POST['fullname'];
        $email=$_POST['emailid']; 
        $mobile=$_POST['mobileno'];
        $password=md5($_POST['password']); 
        $sql="INSERT INTO  tblusers(FullName,EmailId,ContactNo,Password) VALUES(:fname,:email,:mobile,:password)";
        $query = $dbh->prepare($sql);
        $query->bindParam(':fname',$fname,PDO::PARAM_STR);
        $query->bindParam(':email',$email,PDO::PARAM_STR);
        $query->bindParam(':mobile',$mobile,PDO::PARAM_STR);
        $query->bindParam(':password',$password,PDO::PARAM_STR);
        $query->execute();
        $lastInsertId = $dbh->lastInsertId();
        if($lastInsertId) {
            echo "<script>alert('Registration successfull. Now you can login');</script>";
        } else {
            echo "<script>alert('Something went wrong. Please try again');</script>";
        }
    }
?>

<script>
function checkAvailability() {
    $("#loaderIcon").show();
    jQuery.ajax({
        url: "check_availability.php",
        data:'emailid='+$("#emailid").val(),
        type: "POST",
        success:function(data){
            $("#user-availability-status").html(data);
            $("#loaderIcon").hide();
        },
        error:function (){}
    });
}
</script>

<script type="text/javascript">
function valid() {
    if(document.signup.password.value!= document.signup.confirmpassword.value) {
        alert("Password and Confirm Password Field do not match  !!");
        document.signup.confirmpassword.focus();
        return false;
    }
    return true;
}
</script>

<div class="modal fade" id="signupform">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-header">
        <button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">&times;</span></button>
        <h3 class="modal-title">Sign Up</h3>
      </div>
      <div class="modal-body">
        <div class="row">
          <div class="signup_wrap">
            <div class="col-md-12 col-sm-6">
              <form  method="post" name="signup" onSubmit="return valid();">
                <div class="form-group">
                  <input type="text" class="form-control" name="fullname" placeholder="Full Name" required="required">
                </div>
                      <div class="form-group">
                  <input type="text" class="form-control" name="mobileno" placeholder="Mobile Number" maxlength="10" required="required">
                </div>
                <div class="form-group">
                  <input type="email" class="form-control" name="emailid" id="emailid" onBlur="checkAvailability()" placeholder="Email Address" required="required">
                   <span id="user-availability-status" style="font-size:12px;"></span> 
                </div>
                <div class="form-group">
                  <input type="password" class="form-control" name="password" placeholder="Password" required="required">
                </div>
                <div class="form-group">
                  <input type="password" class="form-control" name="confirmpassword" placeholder="Confirm Password" required="required">
                </div>
                <div class="form-group checkbox">
                  <input type="checkbox" id="terms_agree" required="required" checked="">
                  <label for="terms_agree">I Agree with <a href="#">Terms and Conditions</a></label>
                </div>
                <div class="form-group">
                  <input type="submit" value="Sign Up" name="signup" id="submit" class="btn btn-block">
                </div>
              </form>
            </div>
            
          </div>
        </div>
      </div>
      <div class="modal-footer text-center">
        <p>Already got an account? <a href="#loginform" data-toggle="modal" data-dismiss="modal">Login Here</a></p>
      </div>
    </div>
  </div>
</div>

source https://stackoverflow.com/questions/67942201/new-password-and-confirm-password-field-do-not-match-error