Data gagal tersimpan (SOLVED)

saya ingin menyimpan data tapi error muncul alert "Data gagal di tambah" kesalahannya dimana ya

controller

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class Kembali extends CI_Controller {
	public function __construct()
	{
		parent::__construct();
		$this->load->model('Kembali_models');
		$this->load->model('Transaksi_models');
		$this->load->library('form_validation');
		 is_loggedin();
	}

	public function index()
	{
		$data['user'] = $this->db->get_where('user',['username'=>$this->session->userdata('username')])->row_array();
		$data['judul'] 		= "Data Peminjaman";
		$data['transaksi']	= $this->Kembali_models->getAllTrans();
		$this->load->view('templates/header',$data);
		$this->load->view('templates/aside',$data);
		$this->load->view('kembali/index',$data);
		$this->load->view('templates/footer');
	}

	public function denda($id_transaksi){
		$data['user'] = $this->db->get_where('user',['username'=>$this->session->userdata('username')])->row_array();
		$data['transaksi']		= $this->Transaksi_models->getTransaksiById($id_transaksi);
		$data['tanggal_kembali']  = date('Y-m-d');
		$data['judul'] 		= 'Detail denda';
		$this->load->view('templates/header',$data);
		$this->load->view('templates/aside',$data);
		$this->load->view('kembali/denda');
		$this->load->view('templates/footer');
	}

	public function simpan(){
		$data1 = [
			"idtransaksi"   => $this->input->post('id_transaksi'),
            "idbuku"    => $this->input->post('kode_buku'),
            "nis"           => $this->input->post('nis'),
            "nama"          => $this->input->post('nama'),
            "tglkembali"    => $this->input->post('tanggal_kembali'),
            "status_denda"          => $this->input->post('status_denda'),
            "keterangan"            => $this->input->post('keterangan')
		];
		$this->Kembali_models->simpan($data1);
		$result = $this->db->affected_rows();
		if($result > 0){
			echo "<script>
			alert('Data berhasil di tambah');
			window.location.href='".base_url().'kembali'."';
			</script>";
		} else {
			echo "<script>
			alert('Data gagal di tambah');
			window.location.href='".base_url().'kembali'."';
			</script>";
		}
	}
}

model

<?php
class Kembali_models extends CI_model{
	public $table = 'tb_transaksi';
	public function getAllTrans(){
		return $this->db->get('tb_transaksi')->result_array();
	}

public function simpan($data1){
		$this->db->insert('tb_pengembalian',$data1);
	}
}

view

<div class="content-wrapper">
	<div class="panel-body">
		<!-- <form action="" method="POST"> -->
			<?php echo validation_errors(); ?>
			<?php echo form_open('Kembali/simpan'); ?>

			<div class="form-horizontal">
				<div class="box-body">
					<div class="form-group">
						<label class="col-sm-2 control-label">ID Transaksi</label>
						<div class="col-sm-10">
							<input type="text" class="form-control" id="idtransaksi" name="idtransaksi" value="<?= $transaksi['id_transaksi'];?>"
							readonly >
						</div>
					</div>
				</div>
			</div>

			<div class="form-horizontal">
				<div class="box-body">
					<div class="form-group">
						<label class="col-sm-2 control-label">ID Buku</label>
						<div class="col-sm-10">
							<input type="text" class="form-control" id="idbuku" name="idbuku" value="<?= $transaksi['id_buku'];?>" readonly>
						</div>
					</div>
				</div>
			</div>

			<div class="form-horizontal">
				<div class="box-body">
					<div class="form-group">
						<label class="col-sm-2 control-label">Nama</label>
						<div class="col-sm-10">
							<input type="text" class="form-control" id="nama" name="nama" value="<?= $transaksi['nama']?>" readonly>
						</div>
					</div>
				</div>
			</div>

			<div class="form-horizontal">
				<div class="box-body">
					<div class="form-group">
						<label class="col-sm-2 control-label">Tanggal Kembali</label>
						<div class="col-sm-10">
							<input type="text" class="form-control" id="tglkembali" name="tglkembali" value="<?= $tanggal_kembali?>" readonly>
						</div>
					</div>
				</div>
			</div>

      <div class="form-horizontal">
				<div class="box-body">
					<div class="form-group">
						<label class="col-sm-2 control-label">Status Denda</label>
						<div class="col-sm-10">
              <select class="form-control select2" name="status_denda" id="status_denda">
                <option value="nominal">Denda Nominal</option>
                <option value="ganti">Ganti Buku</option>
              </select>
						</div>
					</div>
				</div>
			</div>

      <div class="form-horizontal">
				<div class="box-body">
					<div class="form-group">
						<label class="col-sm-2 control-label">Keterangan</label>
						<div class="col-sm-10">
							<input type="text" class="form-control" id="keterangan" name="keterangan">
						</div>
					</div>
				</div>
			</div>

			<div class="col-sm-1">
				<button type="submit" name="simpan" class="btn btn-primary">Simpan</button>
			</div>
		</form>
	</div>
</div>