Gary Illyes from Google said on Reddit that for a page to be indexed by Google "the content still has to pass quality checks." If that content does not pass the quality checks, Google may not index it even if you manually try to submit the page to Google for indexing.
A webmaaster on Reddit was complaining that the Google Search Console request indexing feature was "not working." So Gary chimed in that it does work but even if you "request" the indexing, Google might not index the page. Gary explained "mind you, it's not a guarantee. the content still has to pass quality checks to get selected for indexing."
So the webmaster asked what he can do to get indexed and Gary said focus on improving the quality. He said "well, yeah, work on the quality. like... a lot. perhaps chat up a few friends who don't mind hurting your feelings and ask them for their opinion, and listen to them."
If you want to use the request indexing feature, then use the URL inspection tool in Google Search Console. It looks like this, you enter a URL into the URL inspection tool and then click the "request indexing" button:
Google then validates the URL can be indexed:
If so, Google will let you know if it is in the queue:
Forum discussion at Reddit.
