404 Error code on API callback for one of the astronauts

There seem to be to issues here: first, the list is adding a period to the name of one of the people, and this causes the Wiki lookup for that person to fail.

But also, if you look up the Wiki pages by hand, you'll find that for that same person (Sergey Korsakov) you get a disambiguation page instead of the info page the code is expecting. This is because there are two people with similar names in the database.

Adding code to detect and remove stray punctuation would be reasonably simple, but to detect and resolve a disambiguation page would be a much more advanced challenge. But as a quick fix for both issues, you could just check for "Sergey Korsakov." (with the period), and substitute "Sergey Korsakov (cosmonaut)":

btn.addEventListener('click', () => {
    getJSON(astrosUrl,(json) => {
    json.people.map( person => {
        if (person.name == "Sergey Korsakov.")  // change bad name to correct one
            getJSON(wikiUrl + "Sergey Korsakov (cosmonaut)", generateHTML);
        else
            getJSON(wikiUrl + person.name, generateHTML);
    });
});