I'm trying to make a call for get To get all my "subcategories", I get the error:
Call of an indefinite relationship (subcategory) on the model (App Sub_Category).

I am in doubt since I do not know if the error is 100% of the database or the query.

Please if someone could help me, I would appreciate it

Classroom

public function store(Request $request) {
    // Recoger los datos
    $json = $request->input('json', null);
    $params = json_decode($json);
    $params_array = json_decode($json, true);
    if (!empty($params_array)) {

        $jwtAuth = new JwtAuth();
        $token = $request->header('Authorization', null);
        $user = $jwtAuth->checkToken($token, true);

        // Validar los datos
        $validate = Validator::make($params_array, (
                    'name' => 'required'
        ));
        //Guardar los datos
        if ($validate->fails()) {
            $data = (
                'code' => 400,
                'status' => 'error',
                'message' => 'Envia el nombre'
            );
        } else {
            $sub_category = new Sub_Category();
            $sub_category->name = $params_array('name');
            $sub_category->category_id = $params->category_id;
            $sub_category->save();

            $data = (
                'code' => 200,
                'status' => 'success',
                'sub_category' => $sub_category
            );
        }
    } else {
        $data = (
            'code' => 400,
            'status' => 'error',
            'mensage' => 'No has enviado ninguna sub_categoria'
        );
    }

    // Devolcer Sub_Categoria
    return response()->json($data, $data('code'));
}

Model

class Sub_Category extends Model
{
    // Referencia a la base de datos
    protected $table = 'sub_categories';

    // Unos a muchos
    public function give_posts(){
        return $this->hasMany('AppPost');
    }

    // Relación Muchos a uno
    public function category(){
        return $this->belongsTo('AppCategory', 'category_id');
    }

Database

The objective is to be able to obtain all the data to be able to modify them later, I leave here the 2 tables to be connected.

CREATE TABLE categories(
id                  int(255) auto_increment not null,
name                varchar(255)  NOT NULL,
created_at           datetime DEFAULT NULL,
updated_at           datetime DEFAULT NULL,
CONSTRAINT  pk_users PRIMARY KEY(id)
)ENGINE=InnoDb;

CREATE TABLE sub_categories(
id                  int(255) auto_increment not null,
name                varchar(50)  NOT NULL,
category_id         int(255) NOT NULL,
created_at           datetime DEFAULT NULL,
updated_at           datetime DEFAULT NULL,
CONSTRAINT  pk_users PRIMARY KEY(id),
CONSTRAINT  fk_post_category FOREIGN KEY(category_id) REFERENCES categories(id)
)ENGINE=InnoDb;