Used basic css and Css flexbox

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

HTML

In order to fix the accessibility issue:

• You need to replace <div class="container"> with the <main class="container"> 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. You can replace your <h2>Improve your front-end skills by building projects</h2> element with the <h1>Improve your front-end skills by building projects</h1> element.

CSS

• If you want to center it correctly, you can use flexbox in the body and also you can use the recommended color for the background in body:

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

• after using flexbox in the body, the .container is updated a little bit

.container {
  background: white;
  max-width: 20rem;
  border-radius: 20px;
  padding: 1.2rem;
  text-align: center;
  font-family: Arial, Helvetica, sans-serif;
}

Hope I am helpful. :)