Solução de componente de código QR usando Flexbox.

1. You need to replace <div class="content__qrcode"> with the <main> tag. Semantic HTML is better for website validation and search engine optimization
2. Each main Header should start with an h1 so the following <h3>Melhore suas habilidades de front-end criando projetos</h3> should be replaced with <h1>Melhore suas habilidades de front-end criando projetos</h1>

Some recommendations regarding your code that could be of interest to you.

HTML

In order to fix the accessibility issues:

• You need to replace <div class="content__qrcode"> with the <main class="content__qrcode"> tag and <div class="attribution"> with the <footer class="content__qrcode"> tag. You'd better use Semantic HTML, and you can also reach more information about it from Using Semantic HTML Tags Correctly.
• Each main content needs to start with an h1 element. Your accessibility report states page should contain a level-one heading. So, you need to use a <h1> element in the <main> tag instead of using <h3>. You can replace your <h3>Melhore suas habilidades de front-end criando projetos</h3> element with the <h1>Melhore suas habilidades de front-end criando projetos</h1> element.

CSS

• If you want to center it easily, you can update body in CSS in this way and remove .container because you don't need it anymore:

body {
  background-color: hsl(212, 45%, 89%);
  font-family: 'Outfit', sans-serif;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
}

• you don't need to use margin in .content__qrcodeanymore because flexbox is used in the body

.content__qrcode {
    background-color: hsl(0, 0%, 100%);
    max-width: 250px;  // max-width makes it responsive
    padding: 20px;
    border-radius: 10px;
}

Hope I am helpful. :)