Тип JSONArray не може бути перетворений у JSONObject

Хоча відповіді можуть обробляти вашу справу, але, дивлячись на ваш JSON, це, мабуть, масив, а не об'єкт, правильний формат вашого JSON може бути таким:

{"id":"1","title":"title","goal":"goal","exercice":"Exercice1","difficulty":"Beginner","duration":"3","image":"..","description":"..."}

Which have no "[", "]" and ",". So here's my solution, if it's expected to receive just a single Object all the time, you should change your API, sending you just an object as I mentioned above, otherwise, that "," at the end is incorrect for an array having just one single item, and anyhow, you have to change your model to except an array [which you can declare it as a list like: List myModelList] so that the parser will work fine.

Спробуйте

 JsonArrayRequest jsonArrayRequest = new JsonArrayRequest(
                    Request.Method.GET,
                    url,
                    null,
                    new Response.Listener() {
                        @Override
                        public void onResponse(JSONArray response) {

                            for(int i = 0; i < response.length(); i++) {
                            JSONObject w = response.getJSONObject(i);

                            String title = w.getString("title");
                            String goal = w.getString("goal");

                            txtv.append(title + ", " + goal +"\n\n");
                             }

                            }catch (JSONException e){
                                e.printStackTrace();
                            }
                        }
                    },
                    new Response.ErrorListener(){
                        @Override
                        public void onErrorResponse(VolleyError error){
                           //Do something when error occurred

                        }
                    }
            );