The theme uses custom post types, and to ensure proper working and enable functionality most of the pages are coded to be divided into two types of content namely single pages and categories.
The effect of this is that in any permalink that you create WordPress will first look for a category and then for a page in creating the permalink.
The error is usually due to the fact that you are using a permalink structure that is incompatible with the theme structure.
The best permalink to use with this theme is:
After you’ve set your permalinks and saved the permalinks should be working fine afterwards.