Why my code doesn't stores the variables in mySQL?

Question

Well, i've been trying, to store the content that the user writes in the input text, and store the id of the checkboxs that the user clicks. I achieved store both things, but each one apart, o better say in other php file. The problem appears when i try to sotres both things using only one php file. Here is my code

HTML

<label for="Teachers_Name">Teacher's Name</label>
<input type="text" name="Teachers_Name" id="Teachers_Name" placeholder="Name" required>
<br>
<br>
<label for="School_Name">School Name</label>
<input type="text" name="School_Name" id="School_Name" placeholder="School Name" required>
<br>
<br>
<label for="Implementation_Quality">Write here your 
Implementation Quality score :</label>
<input type="text" name="Implementation_Quality" id="Implementation_Quality" placeholder="Implementation Quality" required>
</div>
<input type="submit" name="submit" id="submit" value="Send" 
    style="margin-left: 50px; margin-bottom: 20px; margin-top: 
    20px;background:#0774D9; color: #fff; font-size: 20px; 
    border-radius: 10px; font-family: 'Verdana';padding: 5px 
    15px 5px 20px">
<p>BOOKS</p>
<div class="checkbox">
    <br>
    <input type="checkbox" value="20" id="CourseBooks" class="get_value" style="display: none;">
    <label for="CourseBooks">CourseBooks</label>
    <input type="checkbox" value="20" id="PracticeBooks" class="get_value"  style="display: none;">
    <label for="PracticeBooks">PracticeBooks</label>
    <br>
</div>
<h4 id="result"></h4>

Jquery

<script>
    $(document).ready(function() {
        $('#submit').click(function() {
            var insert = [];
            $('.get_value').each(function() {
                if ($(this).is(":checked")) {
                    insert.push($(this).attr("id"));
                }
            });
            insert = insert.toString();
            $.ajax({
                url: "insert1.php",
                method: "POST",
                data: {
                    insert: insert
                },
                success: function(data) {
                    $('#result').html(data);
                }
            });
        });
    });
</script>

Php of checkboxs

if(isset($_POST['insert'])) {
    $conn = mysqli_connect("localhost", "root", "", "datos1");
    $query = "INSERT INTO BOOKS(name) VALUES('" . $_POST["insert"] . "') ";
    $result = mysqli_query($conn, $query);
    echo "Data Inserted Succesfully";
}

PHP of texts

$conexion = new mysqli("localhost", "root", "");

if(!$conexion) {
    echo "Conexión no exitosa";
} else {

    $base = mysqli_select_db($conexion, "datos1");
    if(!$base) {
        echo "No se pudo conectar a la base de datos";
    }
}
//LLAMAMOS LAS VARIBALES
$Teachers_Name = "";
$Teachers_Name = isset($_POST['Teachers_Name']) ? $_POST['Teachers_Name'] : '';
$Teachers_Name = empty($_POST['Teachers_Name']) ? $_POST['Teachers_Name'] : '';
$Teachers_Name= $_POST['Teachers_Name'] ?? '';
$School_Name = "";
$School_Name = isset($_POST['School_Name']) ? $_POST['School_Name'] : '';
$School_Name = empty($_POST['School_Name']) ? $_POST['School_Name'] : '';
$School_Name= $_POST['School_Name'] ?? '';
$Implementation_Quality = "";
$Implementation_Quality = isset($_POST['Implementation_Quality']) ? $_POST['Implementation_Quality'] : '';
$Implementation_Quality = empty($_POST['Implementation_Quality']) ? $_POST['Implementation_Quality'] : '';
$Implementation_Quality= $_POST['Implementation_Quality'] ?? '';
//Guarda las variables
$sql = "INSERT INTO datos_1 (Teachers_Name, School_Name, 
     Implementation_Quality) VALUES(?, ?, ?)";

$sth = mysqli_prepare($conexion, $sql);
mysqli_stmt_bind_param($sth, 'sss', $Teachers_Name, $School_Name, $Implementation_Quality);
$ejecutar = mysqli_stmt_execute($sth);

if(!$ejecutar) {
    echo "Hubo algun error";
} else {
    echo "Datos guardados correctamente<br><a href='index.php'>Volver</a>";
}