Google’s John Mueller said in a webmaster hangout that it is important that the pages on your website are linked from other pages of your website. You should make sure that you do have internal links pointing at your web pages. Otherwise, Google might ignore them:
If you have pages that don’t have internal links pointing at them, so pages that are hard to find from crawling, then usually Google Search will assume that these are not very critical for your website, because you’re essentially hiding away from people who are clicking around within your website.
And if we assume that you think they’re not very critical for your website, then probably we won’t give them as much weight in Search. And if we don’t give them as much weight in Search, then it doesn’t really matter that much what you actually have on those pages. […]
If you have pages within your site that you’re not linking to at all, then we don’t really know what you want to tell us with that. And if those pages are duplicate or low-quality content and we don’t give them much weight, then it doesn’t really matter that much.
On the other hand, if you want those pages to be findable in search, then I would definitely make sure that you do have internal links pointing at them and that you do try to avoid duplicate and low-quality content on those pages.”
You can view the video here:
How to check if your web pages are linked correctly
Use the website audit tool in SEOprofiler to check if the pages on your website are linked correctly. If the website audit tool cannot find the pages on your website, chances are that Google will not list your pages: