Hi studiomediaweb,
Please check the plugin version and the folder permissions (it is beter to set them to 644 or 755). You should create both files manually (robots.txt and sitemap.xml) and set permissions to 755 for them. https://codex.www.ads-software.com/Changing_File_Permissions#Shared_Hosting_with_suexec.
Then go to plugin settings page and mark the checkbox “I want to create a new sitemap file or update the existing one”, and if necessary, mark the checkbox “I want to add sitemap file path in robots.txt” and click Save Changes.
Regards,
BestWebSoft Support Team.