The page tells search engines not to index it

A noindex directive, in a robots meta tag or an X-Robots-Tag response header, removes the page from search results entirely, however good the rest of it is. It is the classic launch mistake: the staging site's noindex goes live with the new design, and traffic falls away over the following weeks with nothing visibly broken.

Source: Block Search indexing with noindex. It is an error when the page is in your sitemap or is the home page, where it can't be meant; otherwise a warning, because thank-you pages and internal search results are often noindexed on purpose.

If the page should be found in search, remove the directive. In the HTML it looks like this:

<meta name="robots" content="noindex" />

In WordPress it is Settings → Reading → Discourage search engines; SEO plugins also set it per page. If we found it in the X-Robots-Tag header, it is set in your server or CDN configuration. If the page is meant to stay out of search, leave it, and take the page out of your sitemap.