CodeIgniter query with appended string containing the WHERE clause emits MySQL error

Put a space before you concatenate the WHERE clause:

....on a.id_user=c.id_user' .$id_kategori);

to

....on a.id_user=c.id_user ' .$id_kategori);

In Controller

public function do_update(){    

    $res = $this->mymodel->UpdateData();

    if($res == TRUE)
    {
        # Success
        $this->session->set_flashdata('pesan','Update Data Sukses');
        redirect('crud/index');
    }
    else
    {
        # Error
        echo "Update Failed";
    }
}

In Model

public function UpdateData(){

    $id_kategori    = $_POST['id_kategori'];
    $nama           = $_POST['nama_lengkap'];
    $judul          = $_POST['judul'];
    $nama_kategori  = $_POST['nama_kategori'];
    $isi            = $_POST['isi'];

    $data = array(
       'nama_lengkap'   => $nama,
       'judul'          => $judul,
       'nama_kategori'  => $nama_kategori,
       'isi'            => $isi
    );

    $this->db->where('id_kategori', $id_kategori);

    if(!$this->db->update('artikel', $data))
    {
        #Error
        return FALSE;
    }
    else
    {
        #Success
        return TRUE;
    }
}

• In a CodeIgniter application, $_POST should not be directly accessed. $this->input->post() is the method of choice.
• For a professional-looking, reliable, PSR-12 compliant codebase, method names should be in camelCase, method parameters should implement type hinting, lines of code should not be excessively wide, and methods' opening and closing curly braces should be on lines by themselves.
• edit_data() is a vague and almost unintuitive endpoint for a controller method that merely presents a form (doesn't actually edit anything). Because this controller might be sensibly called Artikel, it may be enough to call this method loadForm() or something similar.
• The do_update() method name could be simplified to update() and be just as informative.
• It is very bad practice to send a whole query clause expression as a string to the model method. You may enjoy the flexible alternative of passing an associative array as a array $where = [] parameter.
• Use CodeIgniter's active record syntax to build a query with chainable and intuitive methods which help with value and identifier quoting.

Ultimately your script only fails because you smashed your WHERE clause string too close to the end of your query string. It would have been rendered as:

...LEFT JOIN user as c on a.id_user=c.id_userwhere id_kategori =...

but your user table doesn't have a column called id_userwhere.

To put you on a better path, Let me recommend a new Artikel model method which receives an associative array containing WHERE clause conditions.

public function getAll(array $where = []): array
{
    return $this->db
        ->select([
            'a.id_artikel',
            'a.judul',
            'a.tanggal_buat',
            'a.tanggal_update',
            'b.nama_kategori',
            'c.nama_lengkap',
            'c.id_user'
        ])
        ->from('artikel a')
        ->join('kategori b', 'id_kategori', 'left')
        ->join('user c', 'id_user', 'left')
        ->where($where)
        ->get()
        ->result_array();
}

I will mention that I don't understand why you are only accessing the first row of a result set which may contain more than one row. And your provided script is trying to access $mhs which is never declared before it is accessed.