Hugo does not support related posts out of the box. There have been discussions about it but it has not been realized yet. However there are several solutions in the forum.
For example I have slightly modified a snippet that I found at the topic: Using intersect to find related content to render related content based on tags (instead of a custom parameter).
Here is the working code (it goes in single.html)
{{ $page_link := .Permalink }} {{ $tags := .Params.tags }} {{ range .Site.Pages }} {{ $page := . }} {{ $has_common_tags := intersect $tags .Params.tags | len | lt 0 }} {{ if and $has_common_tags (ne $page_link $page.Permalink) }} <---- html here ----> {{ end }} {{ end }}