Resolução para o desafio 1

Para a tag main, foi adicionado as instruções: display: flex, justify-content: center e align-items: center.

Para a tag div, foi adicionado: display: flex, justify-content: center, align-items: center e alterado o padding.

body {
    background-image: url("https://alura-start-public.s3.amazonaws.com/alura%2B/bg---alura-.png");
    background-repeat: no-repeat;
    background-size: cover;
    font-family: 'Fjalla One', sans-serif;
}

main {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    width: 100vw;
}

div {
    display: flex;
    justify-content: center;
    align-items: center;
    width: 240px;
    height: 110px;
    font-size: 36px;
    font-weight: bold;
    background-color: #fff;
    border-radius: 4px;
    padding: 10px;
    background-image: linear-gradient(315deg, #4dccc6 0%, #96e4df 74%);
    box-shadow: 4px 4px 6px 0 rgba(255, 255, 255, .3),
        -4px -4px 6px 0 rgba(116, 125, 136, .2),
        inset -4px -4px 6px 0 rgba(255, 255, 255, .2),
        inset 4px 4px 6px 0 rgba(0, 0, 0, .2);
}