How to activate clean URLs
The clean URLs (permalinks) are mandatory for the theme installation/update
If your WordPress installation uses the default permalinks setting, on the theme configuration panel, you will have this warning:
To fix this problem, simply
click on "Fix it for me" button.
You can also fix it by your own, visiting the WP Admin > Settings > Permalinks and selecting the "Post name" option.
The importance of the clean URLs (permalinks) for SEO
For a better search engine indexing, your installation needs to have a friendly permalink structure. By default, the WordPress use the "plain" setting. A friendly/clean URLs contain keywords that explain the article, and they’re easy to read by both humans and search engines.