how to fix it (A PHP Error was encountered Severity: Warning Message: Undefined array key “kapasitas”

A PHP Error was encountered Severity: Warning Message: Undefined array key “kapasitas” Filename: Admin/data_trip.php Line Number: 75
public function tambah_aksi() {
    // Validasi form
    $this->form_validation->set_rules('nama_gunung', 'Nama Gunung', 'trim|required');
    $this->form_validation->set_rules('harga', 'Harga', 'trim|required|numeric');
    $this->form_validation->set_rules('basecamp', 'Basecamp', 'trim|required');
    $this->form_validation->set_rules('fasilitas', 'Fasilitas', 'trim|required');
    $this->form_validation->set_rules('tanggal', 'Tanggal', 'trim|required');
    $this->form_validation->set_rules('kapasitas', 'Kapasitas', 'trim|required|numeric');

    if ($this->form_validation->run() == false) {
        // Jika validasi gagal, load view 'admin/form_data_trip' untuk menampilkan form input
        $this->load->view('admin/form_data');
    } else {
        // Konfigurasi upload gambar
        $config['upload_path'] = './uploads/';
        $config['allowed_types'] = 'gif|jpg|png';
        $config['max_size'] = 2048;
        $config['encrypt_name'] = TRUE;

        $this->upload->initialize($config);

        if ($this->upload->do_upload('gambar')) {
            $gambar_data = $this->upload->data();
            $gambar = $gambar_data['file_name'];
        } else {
            $gambar = '';
            $upload_error = $this->upload->display_errors('<p class="error">', '</p>');
            $this->session->set_flashdata('upload_error', $upload_error);
        }

        // Tambahkan data trip baru
        $data_trip = array(
            'nama_gunung' => $this->input->post('nama_gunung'),
            'harga' => $this->input->post('harga'),
            'basecamp' => $this->input->post('basecamp'),
            'fasilitas' => $this->input->post('fasilitas'),
            'gambar' => $gambar
        );

        $id_trip = $this->model_trip->tambahTrip($data_trip);

        // Tambahkan jadwal trip
        $data_jadwal = array(
            'id_trip' => $id_trip,
            'jadwal_trip' => $this->input->post('tanggal'),
            'kapasitas' => $this->input->post('kapasitas')
        );

        // Validasi kapasitas
        $current_capacity = $this->model_trip->getCurrentCapacity($id_trip);
        if ($current_capacity < $this->input->post('kapasitas')) {
            $this->model_trip->tambahJadwal($data_jadwal);
            redirect('admin/trip'); // Redirect ke halaman data trip setelah berhasil menambahkan
        } else {
            $this->session->set_flashdata('kapasitas_error', 'Kapasitas melebihi yang tersedia.');
            redirect('admin/trip/tambah');
        }