Codeigniter 3 The upload destination folder does not appear to be writable

I have a Codeigniter 3 project. There was no problem when I was running the site locally, but when I uploaded it to the ngxi server, I got this error: "The upload destination folder does not appear to be writable." My codes are as follows:

public function yazar_update($id) {
    $data = array('isim' => $this->input->post('title'));

    // Yükleme yapılandırması
    $config['upload_path'] = './assets/images/'; // Fotoğrafı kaydedeceğimiz dizin
    $config['allowed_types'] = 'jpeg|jpg|png'; // İzin verilen dosya türleri
    $config['max_size'] = 2048; // Maksimum dosya boyutu (KB)
    $config['encrypt_name'] = TRUE; // Dosya adını rasgele oluştur

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

    // Dosya yükleme işlemi
    if (!$this->upload->do_upload('image')) {
        $update = $this->db->where("yazar_id", $id)->update("yazarlar", $data);
        if ($update) { redirect(base_url("Af45sdieD/edit_yazar/$id")); }else{ print("Olmadı"); }
    } else {
        // Yükleme başarılı
        $upload_data = $this->upload->data();
        $data['yazar_image'] = $upload_data['file_name'];

        $update = $this->db->where("yazar_id", $id)->update("yazarlar", $data);
        if ($update) { redirect(base_url("Af45sdieD/edit_yazar/$id")); }else{ print("Olmadı"); }
    }
}

View code:

<form action="<?= base_url("yazar_update/$yazar->yazar_id"); ?>" method="POST" class="create-form" enctype="multipart/form-data">
                        <h2 class="heading text-center">Yazar Bilgilerini Değiştir</h2>
                        <div class="form-group mb-4">
                            <label for="title">Başlık</label>
                            <input type="text" class="form-control" id="title" name="title" value="<?= $yazar->isim; ?>" required>
                        </div>
                        <div class="form-group mb-4">
                            <label for="title">Kapak Resmi</label>
                            <input type="file" class="form-control" id="image" name="image">
                        </div>
                        <div class="form-group mb-4">
                            <label>Mevcut Kapak Resmi</label>
                            <hr>
                            <img style="width: 200px;" src="<?= base_url("assets/images/$yazar->yazar_image"); ?>" alt="Kapak Resmi" class="img-fluid">
                        </div>
                        <button id="submitBtn2" type="submit" class="btn btn-primary" style="display: block; width: 100%;">Yazar Bilgilerini Güncelle</button>
                    </form>

Note: The permissions of the assets folder are 777 (I'm sure of this). I'm using ngxi server. Please help me or I'll go crazy. Thanks in advance.