body {
  display: flex;
}

.section {
  width: 1260px;
}

.speakers {
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
}

.speaker {
  width: 418px;
  /* Если поставить 420px то 3 столбец съедет вниз, почему так??? Эти 2 пикселя от бордера, почему эти пискели не входят в 420px????*/
  box-sizing: border-box;
  /* Почему не работает????*/
  display: block;
}

<section class="section">
  <h2 class="section-title">Наши эксперты</h2>
  <div class="speakers">

    <div class="speaker">
      <img src="https://netology-code.github.io/html-2-homeworks/block-elements-positioning/our-experts-section/img/6211.jpg" class="speaker-photo" alt>
      <span class="speaker-name">Татьяна Тен</span>
    </div>

    <div class="speaker">
      <img src="https://netology-code.github.io/html-2-homeworks/block-elements-positioning/our-experts-section/img/18702.jpeg" class="speaker-photo" alt>
      <span class="speaker-name">Денис Ежков</span>
    </div>

    <div class="speaker">
      <img src="https://netology-code.github.io/html-2-homeworks/block-elements-positioning/our-experts-section/img/18292.jpeg" class="speaker-photo" alt>
      <span class="speaker-name">Андрей Батусов</span>
    </div>

    <div class="speaker">
      <img src="https://netology-code.github.io/html-2-homeworks/block-elements-positioning/our-experts-section/img/16689.jpeg" class="speaker-photo" alt>
      <span class="speaker-name">Александр Беспоясов</span>
    </div>

    <div class="speaker">
      <img src="https://netology-code.github.io/html-2-homeworks/block-elements-positioning/our-experts-section/img/9998.jpeg" class="speaker-photo" alt>
      <span class="speaker-name">Константин Фокин</span>
    </div>

    <div class="speaker">
      <img src="https://netology-code.github.io/html-2-homeworks/block-elements-positioning/our-experts-section/img/19697.jpeg" class="speaker-photo" alt>
      <span class="speaker-name">Виталий Гусаров</span>
    </div>
  </div>
</section>