This is not an issue generated by the theme, in order to add categories into you URL you'll need to to change your URL structure. In order to do this you'll need to navigate to:
wp-admin => Settings => Permalink
Where you'll find a section like this:

By selecting the Custom Structure option you can modify your URL the way you want, for example:
- Custom Structure - http:// domain.com
/%year%/%monthnum%/%postname%/
In this case, what you need is add the %category% to this input, for example:
- Custom Structure - http:// domain.com
%category%/%postname%/
Please note:
You never, ever put your site url in the permalinks slot. You must use one of the structure tags, or a combination of tags only.
You can use a variety of tags here, check them out on https://codex.wordpress.org/Using_Permalinks