can someone help me by seeing what is wrong with the program I created?

1. Warning: Undefined array key “pertemuan” in D:filexampphtdocsuts_nimkoneksi.php on line 25
2. Fatal error: Uncaught TypeError: Unsupported operand types: string / string in D:Filexampphtdocsuts_nimkoneksi.php:27 Stack trace: #0 D:Filexampphtdocsuts_nimadd_data.php(7): tambah(Array) #1 {main} thrown in D:Filexampphtdocsuts_nimkoneksi.php on line 27

my Code
koneksi.php

function tambah($data) {
    global $conn;

    $nim = htmlspecialchars($data["nim"]);
    $nama = htmlspecialchars($data["nama"]);
    $jurusan = htmlspecialchars($data["jurusan"]);
    $sks = htmlspecialchars($data['sks']);
    $pertemuan = htmlspecialchars($data['pertemuan']);
    $kehadiran = htmlspecialchars($data['kehadiran']);
    $persen = ($kehadiran / $pertemuan) * 100;
    $persen = min($persen, 100);

add_data.php

<div class="col-md-3">
                 <div class="form-floating">
                    <select class="form-select" name="sks" id="sks" onchange="updatePertemuan()" required>
                            <option selected hidden>Pilih SKS</option>
                            <option value="2">2 SKS</option>
                            <option value="3">3 SKS</option>
                            <option value="4">4 SKS</option>
                        </select>
                        <label for="sks">SKS</label>
                    </div>
                </div>
                <div class="col-md-3">
                    <div class="form-floating">
                        <input type="text" name="pertemuan" id="pertemuan" 
                        class="form-control" disabled>
                        <label for="pertemuan">Pertemuan</label>
                    </div>
                </div>

   <script>
        function updatePertemuan() {
            const sks = document.getElementById('sks').value;
            let pertemuan = 0;

            if (sks === '2') {
                pertemuan = 14;
            } else if (sks === '3') {
                pertemuan = 21;
            } else if (sks === '4') {
                pertemuan = 29;
            }

            document.getElementById('pertemuan').value = pertemuan;
        }
    </script>

If there is a file/line of code you would like to see please tell me

I want to add the data to the database using PHP code