Non-indexable pages are pages on your domain that cannot be found in the Search Engine Result Pages (SERPs) because of your domain settings. There are many possible reasons why this could be the case.
If your site is blocked for search engines via robots.txt or is in a blocked directory, the site is considered to be non-indexable. Check your robots.txt file first to make sure that search engines can access your page. More information about the robots.txt file can be found in the Ryte Wiki.
With the Canonical tag you specify search engines which URLs are canonical. This is important to avoid problems with duplicate content. If the canonical tag refers to another page, the page is not indexable. You can find more information about the Canonical tag in the Ryte Wiki.
With a redirection you indicate that the content of a page can be found on another URL and refers the user directly to this URL. The most common redirections are 301 (permanent redirection) and 302 (temporary redirection). A URL that returns 301 (or even 302 in the long term) as status code will not be indexed by search engines. More information about status codes can be found in the Ryte Wiki.