What language is your site?

English (United Kingdom), the only one installed.

Confirmed
i suspect the cause is that $code is equal *
public static function create(string $code): Stemmer

@Hackwar
Could you please look at that?

The stemmer has been updated can you please retest. See #31732

Seems not solved, as I get this

Are you testing with the nightly builds?

https://developer.joomla.org/nightly-builds.html

Just tested with Joomla_4.0.0-beta6-dev-Development-Update_Package.zip from here:

• https://developer.joomla.org/nightly-builds.html

@alikon is right in a multilingual site:

If you var_dump $code

    public static function create(string $code): Stemmer
    {
        $code = UTF8::strtolower($code);
var_dump($code);
        foreach (self::LANGS as $classname => $isoCodes) {
            if (in_array($code, $isoCodes)) {
                return new $classname;
            }
        }

        throw new NotFoundException(sprintf('Stemmer not found for %s', $code));
    }

You get when en-GB is the site language (would be *, fr, fr for French):

`string(1) "*" string(2) "en" string(2) "en" `

If you work in a monolanguage site, then you may have to set Smart Search Options to your language or Default Site Language.

Do you still get the Exception in that case?

Note

I do not get any suggestions when typing in the search field.
Could someone confirm?

@Hackwar

"Information Required" label removed since the requested information if it still happens with current 4.0 has been provided.

The problem is not the exception. That is caught and the indexer still runs. So no need to fix here anything, it's expected behaviour that there is one when the current language has no stemmer.
But the thing is, that if "*" is the current language, then it's not translated in e.g. "en-GB" (= the current language on the site).
#31795 tries to fix it and reduces the exception numbers.

#39959 should fix this. Closing since we have a PR.