I'm Getting Application Form Error

First of all, I wish everyone good form. Thank you very much in advance for your help. My problem is, I am preparing an application form, but I couldn’t get it to work. Where and what kind of mistake am I making? Can you help me? These are my codes.

<?php
$hata = false;
$gonder = false;
if( isset($_POST["islem"]) && $_POST["islem"]=="gonder" ) {

if( !empty($_POST["adsoyad"]) && !empty($_POST["email"]) && !empty($_POST["telefon"]) && !empty($_POST["konu"]) && !empty($_POST["mesaj"]) && !empty($_POST["eht"]) && !empty($_POST["arac"]) && !empty($_POST["tam"]) ) {

include_once('phpmailer/class.phpmailer.php');

$mail = new PHPMailer();
$mail->isSMTP();
$mail->SMTPAuth = true;
$mail->Host = 'mail.deneme.com';
$mail->Username = '[email protected]';
$mail->Password = '';
$mail->Port = 587;
$mail->setFrom('[email protected]', 'deneme');
$mail->addAddress('[email protected]', 'deneme');

//HTML Aktif
$mail->isHTML(true);
$mail->CharSet ="utf-8";
//Mail Başlığı
$mail->Subject = 'İletişim Formu Mesajı';
//Mail İçeriği
$mail->Body = '<p><strong>Gönderen:</strong> ' . $_POST["adsoyad"] . ' - ' . $_POST["email"] . '</p>'.
'<p><strong>Konu:</strong> ' . $_POST["telefon"] . '</p>'.
'<p><strong>Konu:</strong> ' . $_POST["konu"] . '</p>'.
'<p><strong>Mesaj:</strong> ' . $_POST["mesaj"] . '</p>'.
'<p><strong>Ehliyet Seçimi:</strong> ' . $_POST["eht"] . '</p>'.
'<p><strong>Araç Seçimi:</strong> ' . $_POST["arac"] . '</p>'.
'<p><strong>Çalışma Turu:</strong> ' . $_POST["tam"] . '</p>';

//Gönder
if ( $mail->send() ) {
$gonder = true;
} else {
$hata = true;
$hata_mesaj = "Mesaj gönderilirken bir hata oluştu: ".$mail->ErrorInfo;
}
} else {
$hata = true;
$hata_mesaj = "Lütfen tüm alanları doldurun.";
}

}
?>
<form class="loftForm" id="contactForm" method="POST" action="">
<div class="row gp-3">
<div class="col-md-12">
<label class="ns-label">Adınız Soyadınız</label>
<input class="ns-control mb-4" type="text" name="adsoyad" id="adsoyad" placeholder="Adınız Soyadınız" required>
</div>
<div class="col-md-6">
<label class="ns-label">Telefon</label>
<input class="ns-control mb-4" type="telefon" name="telefon" id="telefon" placeholder="Telefon" required>
</div>
<div class="col-md-6">
<label class="ns-label">E-Posta</label>
<input class="ns-control mb-4" type="email" name="email" id="email" placeholder="email" required>
</div>
<div class="col-md-12">
<label class="ns-label">Kaç yıllık tecrübeye sahipsiniz?</label>
<input type="text" class="ns-control mb-4" name="kyts" id="kyts" placeholder="Kaç yıllık tecrübeye sahipsiniz?" required/>
</div>
<div class="col-md-12">
<label class="ns-label">Ehliyet Seçimi</label><br>
<input type="checkbox" name="eht[]" value="A1"/><span style="color: #fdb43f;"> A1</span><br/>
<input type="checkbox" name="eht[]" value="A2"/><span style="color: #fdb43f;"> A2</span><br/>
<input type="checkbox" name="eht[]" value="AB"/><span style="color: #fdb43f;"> A,B</span><br/>
<input type="checkbox" name="eht[]" value="C"/><span style="color: #fdb43f;"> C</span><br/>
<input type="checkbox" name="eht[]" value="CE"/><span style="color: #fdb43f;"> CE</span><br/>
<input type="checkbox" name="eht[]" value="D"/><span style="color: #fdb43f;"> D</span><br/>
<input type="checkbox" name="eht[]" value="DE"/><span style="color: #fdb43f;"> DE</span><br/>
</div>
<div class="col-md-12"><br>
<label class="ns-label">Sahip olduğunuz araç çeşidi</label><br>
<input type="checkbox" name="arac[]" value="motor"/><span style="color: #fdb43f;"> Motor</span><br/>
<input type="checkbox" name="arac[]" value="minivan"/><span style="color: #fdb43f;"> Minivan</span><br/>
<input type="checkbox" name="arac[]" value="kamyon"/><span style="color: #fdb43f;"> Kamyon</span><br/>
<input type="checkbox" name="arac[]" value="kamyonet"/><span style="color: #fdb43f;"> Kamyonet</span><br/>
<input type="checkbox" name="arac[]" value="frigolu"/><span style="color: #fdb43f;"> Frigolu Araç</span><br/>
</div>
<div class="col-md-12"><br>
<label class="ns-label">Tercih ettiğiniz çalışma şekli</label><br>
<input type="checkbox" name="tam[]" value="tamzamanli"/><span style="color: #fdb43f;"> Tam Zamanlı</span><br/>
<input type="checkbox" name="tam[]" value="yarizamanli"/><span style="color: #fdb43f;"> Yarı Zamanlı</span><br/>
</div>
<div class="col-md-12"><br>
<button class="btn comSend anibut mt-2" name="islem" value="gonder" required>
Gönder <span><i class="fas fa-paper-plane"></i></span>
</button>
</div>
</div>
</form>
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