Variable "entry" does not exist in template at Structure Section
That entry variable will only be predefined for the template when you go to one of your entry’s URLs.

Your section’s Entry URL Format for top-level entries is {slug}, and your entry’s slug is qweasdzxc, so that would make your entry’s URL &lt;site-url&gt;/qweasdzxc (http://localhost/qweasdzxc).

If you change your section’s top-level Entry URL Format to reviews/{slug} instead of just {slug}, then the entry’s URL will become &lt;site-url&gt;/reviews/qweasdzxc.

When you go to &lt;site-url&gt;/reviews, you’re just accessing the reviews/index.twig template directly, so the entry variable was never defined, which is causing the error you’re getting.