Data tidak bisa masuk ke database

controller

<?php

/**
 *
 */
class Nilai extends CI_Controller
{

	public function index()
	{
		$data = array(
			'nim'			=> set_value('nim'),
			'id_thn_akad'	=> set_value('id_thn_akad')
		);

		$this->load->view('templates_administrator/header');
		$this->load->view('templates_administrator/sidebar');
		$this->load->view('administrator/masuk_khs',$data);
		$this->load->view('templates_administrator/footer');
	}

	public function nilai_aksi(){
		$this->_rulesKhs();
		if($this->form_validation->run()== FALSE){
			$this->index();
		}else{
			$nim		= $this->input->post('nim',TRUE);
			$thn_akad 	= $this->input->post('id_thn_akad',TRUE);

			$query		= "SELECT krs.id_thn_akad,
							krs.kode_matakuliah,
							matakuliah.nama_matakuliah,matakuliah.sks,
							krs.nilai
							FROM
								krs
							INNER JOIN matakuliah
							ON(krs.kode_matakuliah = matakuliah.kode_matakuliah)
							WHERE krs.nim = $nim AND krs.id_thn_akad = $thn_akad";

			$sql = $this->db->query($query)->result();

			$smt = $this->db->select('tahun_akademik,semester')
				->from('tahun_akademik')
				->where(array('id_thn_akad'=>$thn_akad))->get()->row();

			$query_str = "SELECT mahasiswa.nim,
								 mahasiswa.nama_lengkap,
								 prodi.nama_prodi
						FROM
							mahasiswa
						INNER JOIN prodi
						ON(mahasiswa.nama_prodi = prodi.nama_prodi);";

			$mhs = $this->db->query($query_str)->row();

				if($smt->semester == 1){
					$tampilSemester = "Ganjil";
				}else{
					$tampilSemester = "Genap";
				}

				$data = array(
					'mhs_data'	=> $sql,
					'mhs_nim'	=> $nim,
					'mhs_nama'	=> $mhs->nama_lengkap,
					'mhs_prodi' => $mhs->nama_prodi,
					'thn_akad'	=> $smt->tahun_akademik."(".$tampilSemester.")"
				);

				$this->load->view('templates_administrator/header');
				$this->load->view('templates_administrator/sidebar');
				$this->load->view('administrator/khs',$data);
				$this->load->view('templates_administrator/footer');

		}
	}

	public function _rulesKhs(){
		$this->form_validation->set_rules('nim','nim','required');
		$this->form_validation->set_rules('id_thn_akad','id_thn_akad','required');

	}

	public function input_nilai(){
		$data = array(

			'kode_matakuliah' 		=> set_value('kode_matakuliah'),
			'id_thn_akad'				=> set_value('id_thn_akad')

		);

			$this->load->view('templates_administrator/header');
			$this->load->view('templates_administrator/sidebar');
			$this->load->view('administrator/input_nilai_form',$data);
			$this->load->view('templates_administrator/footer');

	}

	public function input_nilai_aksi(){
		$this->_rulesInputNilai();
		if($this->form_validation->run() == FALSE){
			$this->input_nilai();
		}else{
			$kode_matakuliah = $this->input->post('kode_matakuliah',TRUE);
			$id_thn_akad	 = $this->input->post('id_thn_akad',TRUE);

			$this->db->select('k.id_krs,k.nim,m.nama_lengkap,k.nilai,d.nama_matakuliah');
			$this->db->from('krs as k');
			$this->db->join('mahasiswa as m','m.nim = k.nim');
			$this->db->join('matakuliah as d','k.kode_matakuliah = d.kode_matakuliah');
			$this->db->where('k.id_thn_akad',$id_thn_akad);
			$this->db->where('k.kode_matakuliah',$kode_matakuliah);
			$query = $this->db->get()->result();

			$data = array(
				'list_nilai'		=> $query,
				'kode_matakuliah'	=> $kode_matakuliah,
				'id_thn_akad'		=> $id_thn_akad

			);

				$this->load->view('templates_administrator/header');
				$this->load->view('templates_administrator/sidebar');
				$this->load->view('administrator/form_nilai',$data);
				$this->load->view('templates_administrator/footer');

		}
	}

	public function _rulesInputNilai(){
		$this->form_validation->set_rules('kode_matakuliah','Kode Mata Kuliah','required');
		$this->form_validation->set_rules('id_thn_akad','Tahun Akademik','required');

	}
}
?>

views form_nilai

<?php

$nilai 	= get_instance();
$nilai->load->model('matakuliah_model');
$nilai->load->model('tahunakademik_model');
 ?>

 <div class="container-fluid">

 	<?php
 	if($list_nilai == null){
 		$thn = $nilai->tahunakademik_model->get_by_id($id_thn_akad);
 		$semester = $thn->semester == 1	;

 		if($semester == 1){
 			$tampilSemester = "Ganjil";
 		}else{
 			$tampilSemester = "Genap";
 		}
 	}

 	 ?>

 	 <div class="alert alert-danger">
 	 	Maaf, kode mata kuliah yang anda input <strong>TIDAK TERSEDIA!</strong>di tahun ajaran <?php echo $thn->tahun_akademik . "(".$tampilSemester.")"; ?>
 	 </div>

 	 <?php echo anchor('administrator/nilai/input_nilai','<div class = "btn btn-sm btn-primary">Kembali</div>') ?>
 </div>

views input_nilai_form

<div class="container-fluid">

	<div class="alert alert-success" role="alert">
		<i class="fas fa-university"></i>
		FORM MASUK HALAMAN INPUT NILAI
	</div>

	<form method="post" action="<?php echo base_url('index.php/administrator/nilai/input_nilai_aksi') ?>">

		<div class="form-group">
			<label>Tahun Akadmik (Semester)</label>
			<?php
			$query = $this->db->query('SELECT id_thn_akad, semester, CONCAT(tahun_akademik,"/") AS ta_semester FROM tahun_akademik');
			$dropdowns = $query->result();

			foreach ($dropdowns as $dropdown){
				if($dropdown->semester == 1){
					$tampilSemester = "Ganjil";
				}else {
					$tampilSemester = "Genap";
				}
				$dropDownList[$dropdown->id_thn_akad]= $dropdown->ta_semester." ".$tampilSemester;
			}

			echo form_dropdown('id_thn_akad
				',$dropDownList,'','class ="form-control"'); ?>
		</div>

		<div class="form-group">
			<label>Kode Mata Kuliah</label>
			<input type="text" name="kode_matakuliah" class="form-control" placeholder="Masukan kode matakuliah">
		</div>
		 <button type="submit" class="btn btn-primary">Proses</button>
	</form>
</div>