retrofit Expected a string but was BEGIN_OBJECT at line 1 column 2 path $

.addConverterFactory(ScalarsConverterFactory.create()) .addxxxx

Try like this

 @Override
    public void onResponse(Call<ResponseBody> call, Response< ResponseBody > response) {
        try {
            if (response.body() != null)
                Log.e("UploadResponse>>>", response.body());
            layoutUpload.setVisibility(View.GONE);
            if (file.exists()) {
                file.delete();
                thumb.delete();
            }
        } catch (Exception e) {
            e.printStackTrace();
            layoutUpload.setVisibility(View.GONE);
            if (file.exists()) {
                file.delete();
                thumb.delete();
            }
        }
    }

Error say's you want to get result in String body. If you want to do this, Just add ScalarsConverterFactory.create() in your Retrofit.Builder.

public static Retrofit getClient() {
    Gson gson = new GsonBuilder()
            .setLenient()
            .create();
    Retrofit retrofit = new Retrofit.Builder()
            .baseUrl(getBaseUrl())
            .addConverterFactory(ScalarsConverterFactory.create())
            .addConverterFactory(GsonConverterFactory.create(gson))
            .build();
    return retrofit;
}

Use retrofit Implementation in app level build.gradle.

implementation 'com.squareup.retrofit2:converter-scalars:2.1.0'