Thanks for reaching out for clarity on this @Hundhelhet!
In many cases, a page not being indexed is not necessarily indicative of an issue, rather the page does not meet the parameters set down by Google for indexing, or the indexing of the page has been delayed.
For example, in the case of ‘Discovered - currently not indexed’. The page was found by Google, but the crawl has been rescheduled to prevent the site from being overloaded.
For duplicate pages without a user-selected canonical, Google recognizes the page as a duplicate, and has chosen another page as the canonical to be indexed. If you believe the page has been marked as a duplicate in error, there are steps you can take to rectify this.
You’ll find a full list of reasons for a page not being indexed, alongside the explanation and any actions you might want to take, here.
I hope this helps!
