Introduction to the Google coverage report and interpretation of its data

It also lists the problems faced by Googlebot during crawling and indexing.

Error: The page is not indexed. There can be several explanations for this

Excluded: The page is not indexed, Google has followed the rules of your site such as no-index tag in robots.txt

If there is a sudden increase in errors, you can analyze the data and relate it to the impressions as well.