Nesting block tags doesnt work
Unfortunately it’s not possible for an included template to override the contents of a block in the parent template, or its layout.

You’d need to just override the seo block from page.twig do whatever you need to do from there instead.

{% extends 'layout.twig' %}

{% block main %}
  some content..
  {% include 'pagination.twig' %}
{% endblock %}

{% block seo %}
  &lt;link href="url" rel="canonical"&gt;
{% endblock %}