extractSummary twig filter is unknown I'm trying to set a fallback description in case there is none. This is the bit of code:

{% if seomaticMeta.seoDescription is empty and entry is defined %}
    {% set fallbackDescription = "" %}
    {% if entry.description is defined %}
        {% set fallbackDescription = extractSummary( entry.description ) %}
    {% endif %}
    {% if fallbackDescription is not empty %}
        {% set seomaticMeta = seomaticMeta | merge({
            seoDescription: fallbackDescription,
            og: {
                description: fallbackDescription
            },
            twitter: {
                description: fallbackDescription
            }
        }) %}
    {% endif %}
{% endif %}


But I get an error: Unknown "extractSummary" function in "_globals/_layout".

Am I doing something wrong?
So it sounds to me like either the plugin isn't installed (which doesn't make a ton of sense, because you'd be getting other errors, too), or if it is installed, pieces of it are missing.

extractSummary is a Twig filter/function and has been available since the start; I'd try making sure there's nothing wrong with your install/deploy, or try re-downloading a fresh copy of the SEOmatic plugin.