in one of your past posts but there's no way I can access my sitemap hereThere is already sitemap. The below links are the same:
https://demo.photo.gallery/sitemap.xml
https://demo.photo.gallery/sitemap/
http://gallery.kog.it/sitemap.xml
since google webmaster tell me
Code
We encountered an error while trying to access your Sitemap. Please ensure your Sitemap follows our guidelines and can be accessed at the location you provided and then resubmit.
Network unreachable: Network unreachable
HTTP Error: 500