To find out why an URL isn't indexable, please take a look at the indexability report. There are a number of reasons why an URL might not be indexable:
1. Disallow via robots.txt:
Telling search engine bots via robot.txt file not to crawl your website should be handled with care. Pages that are disallowed via robots.txt will not be indexed, in many cases. But they can be indexed if other pages are referring or linking to them. If you don't want a page to be indexed, please use the canonical tag or the meta tag "robots" and set it to "noindex".
2. The canonical tag refers to another page:
The canonical tag defines which page should be indexed. If your page has a canonical tag to another page, the page using the canonical tag is not indexable.
When a page redirects to another page via 3xx status code the goal of the redirect gets indexed instead.
4. Meta tag "robots":
By setting the meta tag "robots" to "noindex" you can tell search engines that these pages should not be indexed. But be careful: do not use the "robots" tag in combination with a robots.txt disallow.
Paginated pages using next/rel prev usually aren't indexed by search engines.
6. Broken pages:
Pages that answer with header status codes 4xx, 5xx or 9xx also won't get indexed by search engines.