Hi Pham,
This is a known issue, Webinoly comes with Yoast Sitemap pre-configured and this is causing conflicts and issues with other sitemap plugins. It will be fixed in the next major update!
As a temporal fix I suggest you delete the Yoast code:
- Go to: sudo nano /etc/nginx/common/wpcommon.conf
- At the end of the file you will find the Yoast Sitemap code, remove it!
- Restart NGINX: sudo nginx -t && sudo service nginx reload
Now, you can add your sitemap code in your site conf or replacing the Yoast code. Don't forget to restart nginx each time you chage something in your conf.
Please, tell me if it works for you.
Regards.