How to enable a sidebar

Theme: Medium
I searched forum, many topics on sidebar. I just want to activate a simple sidebar.
There is no enable sidebar in the configuration or pages.

you will need to include partials/sidebar.html.twig in your base.html.twig, like so:

{% block sidebar %}
    {% include 'partials/sidebar.html.twig' %}
{% endblock %}

The solution is going to depend which theme you are using. @hoernerfranz's answer may be correct.

yes, I think so, sidebar.html.twig is in the OP's theme's (Medium) partials folder, but it is not included in any other template, so just including it should do the trick.

My apologies, I missed the statement of theme at the top of OP's initial question.

Thank you
I am using the default theme.