Warning: Undefined array key "Days"

Question

i get this warning when i try post data from a checked input to database

Warning: Undefined array key "Days" in C:\xampp\htdocs\oramexhomes\backend\authController.php on line 668

Fatal error: Uncaught TypeError: implode(): Argument #1 ($pieces) must be of type array, string given in

if(isset($_POST['appointment'])){

        $_SESSION['agentid']= 'ueuyy763';
        $checkBox = implode(',', $_POST['Days']);
        $_POST['Days']= array();

        $sql = "INSERT INTO appointment ( Property_number_fk, agent_id, monday, tuesday, wednesday, thursday, friday, saturday, sunday, 8am_9am, 9am_10am, 10am_11am, 11am_12noon, 12noon_1pm, 1pm_2pm, 2pm_3pm, 3pm_4pm, 4pm_5pm, 5pm_6pm, Meetingaddress)
        VALUES (:Property_number, :agent_id, :monday, :tuesday, :wednesday, :thursday, :friday, :saturday, :sunday, :8am-9am, :9am-10am, :10am-11am, :11am-12noon, :12noon-1pm, :1pm-2pm, :2pm-3pm, :3pm-4pm, :4pm-5pm, :5pm-6pm, :Meetingaddress)";
        $stmt = $pdo->prepare($sql);
        $stmt->execute(array(
        ':Property_number' =>  "SELECT Property_number FROM property WHERE agent_id = :agentid",
        ':agent_id' => $_SESSION['agentid'],
        ':monday' => $_POST['Days'][0],
        ':tuesday' => $_POST['Days'][1],
        ':wednesday' => $_POST['Days'][2],
        ':thursday' => $_POST['Days'][3],
        ':friday' => $_POST['Days'][4],
        ':saturday' => $_POST['Days'][5],
        ':sunday' => $_POST['Days'][6],
        ':8am-9am' => $_POST['8am-9am'],
        ':9am-10am' => $_POST['9am-10am'],
        ':10am-11am' => $_POST['10am-11am'],
        ':11am-12noon' => $_POST['11am-12noon'],
        ':12noon-1pm' => $_POST['12noon-1pm'],
        ':1pm-2pm' => $_POST['1pm-2pm'],
        ':2pm-3pm' => $_POST['2pm-3pm'],
        ':3pm-4pm' => $_POST['3pm-4pm'],
        ':4pm-5pm' => $_POST['4pm-5pm'],
        ':5pm-6pm' => $_POST['5pm-6pm'],
        ':Meetingaddress' => $_POST['meeting'],
        )); }

enter code here

html code

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`enter code here`

Days and Time available for inspection

Please, select days and dates you are sure you will be available for inspection. Bear in mind that not keeping to these selected appointment will make our client unhappy.

       <table class="table table-striped">
        <thead>   
        <tr>
    <th> Days</th>
    <th> Time</th>
            </tr>
        </thead>
        <tbody>

            <tr>
        <td>
    
            <label for="Monday">Monday</label>
        <input type="checkbox" id="Monday" name= "Days[]" value="1">
        
        </td>
        <td>
        <label for="8am-9am">8am-9am</label>
        <input type="checkbox" id="8am-9am" name="8am-9am" value="1"><br>
        </td>
            </tr>
           
            <tr>
                <td>
                <label for="Tuesday">Tuesday</label>
                <input type="checkbox" id="Tuesday" name="Days[]" value="1">
                </td>

                <td>
                <label for="9am-10am">9am-10am</label>
                <input type="checkbox" id="9am-10am" name="9am-10am" value="1">
                </td>
            </tr>

            <tr>
                <td>
                <label for="Wednesday">Wednesday</label>
                <input type="checkbox" id="Wednesday" name="Days[]" value="1">
                </td>

                <td>
                <label for="10am-11am">10am-11am</label>
                <input type="checkbox" id="10am-11am" name="10am-11am" value="1">
                </td>
            </tr> <tr>
                <td>
                <label for="Thursday">Thursday</label>
                <input type="checkbox" id="Thursday" name="Days[]" value="1">
                </td>

                <td>
                <label for="11am-12noon">11am-12noon</label>
                <input type="checkbox" id="11am-12noon" name="11am-12noon" value="1">
                </td>
            </tr> <tr>
                <td>
                <label for="Friday">Friday</label>
                <input type="checkbox" id="friday" name="Days[]" value="1">
                </td>

                <td>
                <label for="12noon-1pm">12noon-1pm</label>
                <input type="checkbox" id="12noon-1pm" name="12noon-1pm" value="1">
                </td>
            </tr> <tr>
                <td>
                <label for="Saturday">Saturday</label>
                <input type="checkbox" id="Saturday" name="Days[]" value="1">
                </td>

                <td>
                <label for="1pm-2pm">1pm-2pm</label>
                <input type="checkbox" id="1pm-2pm" name="1pm-2pm" value="1">
                </td>
            </tr> <tr>
                <td>
                <label for="Sunday">Sunday</label>
                <input type="checkbox" id="Sunday" name="Days[]" value="1">
        
                </td>

                <td>
                <label for="2pm-3pm">2pm-3pm</label>
                <input type="checkbox" id="2pm-3pm" name="2pm-3pm" value="1">
                </td>
            </tr>