1- uninstall your plugin sitemap generator.
2- excludes it from its folder.
3- download this file that is on the right side of the site ( Download Zip) https://github.com/navjottomer/sitemap_plugin
4- unzip the folder to your desktop.
5- rename the plugin folder (plugin-sitemap_generator-master) for (sitemap_plugin) - if you do not rename will not work
6- delete your sitemap.xml file to your root folder. do not forget the one who is in the public folder.
7- add sitemap_plugin inside the plugin folder, then install it by adm panel and ready
8- to manage plugin once installed, click Configure, and set your taste!