How to get rid of overlaying

The main problem is that the header is fixed and thus overlaying the main part of the web page. So what I did is just margin-top to the main part, which appears to be not working. Moreover, if you will deleted header, it can be seen that the main section just goes too up.

Overview: image|| without header

HTML:

    <header class="header">
    <nav class="navbar">
        <ul class="navbar-nav">
            <li class="nav-item"><a class="nav-link" href="index.html">Visualizer</a></li>
            <li class="nav-item"><a class="nav-link" href="instruction.html">Instructions</a></li>
            <li class="nav-item active"><a class="nav-link" href="credits.html">Credits</a></li>
        </ul>

    </nav>
</header>

<main class="credits">
    <div class="">

        <div class="cr-card">
            <div class="cr-img">
                <img src="img/p1.png">
            </div>

            <div class="cr-info">

                <h1 class="cr-name">
                    Lorem
                </h1>
                <p class="cr-about">
                    lorem ipsum
                </p>
                <ul class="cr-links">
                    <li>
                        <a href="https://github.com/lorem"><i class="fab fa-github"></i></a>
                    </li>
                    <li>
                        <a href="https://t.me/lorem"><i class="fab fa-telegram"></i></a>
                    </li>
                    <li>
                        <a href="https://www.instagram.com/lorem/"><i class="fab fa-instagram"></i></a>
                    </li>
                </ul>

            </div>
        </div>
        <div class="cr-card">
            <div class="cr-img">
                <img src="img/p1.png">
            </div>

            <div class="cr-info">

                <h1 class="cr-name">
                    Lorem
                </h1>
                <p class="cr-about">
                    lorem ipsum
                </p>
                <ul class="cr-links">
                    <li>
                        <a href="https://github.com/lorem"><i class="fab fa-github"></i></a>
                    </li>
                    <li>
                        <a href="https://t.me/lorem"><i class="fab fa-telegram"></i></a>
                    </li>
                    <li>
                        <a href="https://www.instagram.com/lorem/"><i class="fab fa-instagram"></i></a>
                    </li>
                </ul>

            </div>
        </div>
        <div class="cr-card">
            <div class="cr-img">
                <img src="img/p1.png">
            </div>

            <div class="cr-info">

                <h1 class="cr-name">
                    Lorem
                </h1>
                <p class="cr-about">
                    lorem ipsum
                </p>
                <ul class="cr-links">
                    <li>
                        <a href="https://github.com/lorem"><i class="fab fa-github"></i></a>
                    </li>
                    <li>
                        <a href="https://t.me/lorem"><i class="fab fa-telegram"></i></a>
                    </li>
                    <li>
                        <a href="https://www.instagram.com/lorem/"><i class="fab fa-instagram"></i></a>
                    </li>
                </ul>

            </div>
        </div>
    </div>
</main>

SCSS:

.header {
margin-top: 0px;
top: 0;
width: 100%;
height: $header-height;
z-index: 5;
background: radial-gradient(at center top, rgb(46, 51, 58) 0, rgba(27, 30, 34, 0) 80%);
border: 1px;

.navbar {
    width: 100%;
    margin-right: auto;
    margin-left: auto;
    display: flex;
    flex-flow: row nowrap;
    justify-content: center;
    align-items: center;



    .navbar-nav {
        display: flex;
        flex-flow: row nowrap;
        align-items: center;
        justify-content: center;

        .nav-item {
            display: flex;
            align-items: center;
            justify-content: center;
            color: #ffffff;
            cursor: pointer;

            .nav-link {
                font-family: "Raleway", "Hack", Arial, sans-serif;
                font-size: 1.1em;
                color: white;
                height: 68px;
                line-height: 68px;
                padding: 0 15px;
                border-bottom: 0px #3d5cff solid;

                &:hover {
                    border-bottom: 5px #3d5cff solid;
                }
            }

            &.active {
                .nav-link {
                    border-bottom: 5px #fff solid;
                }
            }
        }
    }




}

}