您可以重写以容器.container-fluid.menu为目标的css{display:flex；justify-content:center；}

null

.container-fluid.menu {
    display: flex;
     justify-content: center;
}
/*menu bar style*/
.menu{
    width:100%;
    position: relative;
    color: black;
}

.my-navbar{

    position: absolute;
    z-index:999;
    width: 90vw;
    height: 10vh;
    padding: 20px 20px 20px 20px;
    background-color: white;
    top:20px;
    box-shadow:0px 4px 15px rgba(0,0,0,0.4);
    z-index:999;
}
.navbar-brand img{
    position: relative;
    width: 10vw;
    height: 15vh;
    margin: 0;
    float: none;
}

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">
<div class="container-fluid menu">
    <nav class="navbar navbar-expand-lg navbar-light my-navbar">

        <a class="navbar-brand" href="#">
          <img src="https://cdn.pixabay.com/photo/2017/01/14/12/59/iceland-1979445_960_720.jpg"  alt="" loading="lazy">
        </a>

        <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarNav" aria-controls="navbarNav" aria-expanded="false" aria-label="Toggle navigation">
        <span class="navbar-toggler-icon"></span>
        </button>

        <div class="collapse navbar-collapse" id="navbarNav">
            <ul class="navbar-nav mr-auto">
                <li class="nav-item active">
                <a class="nav-link" href="#">About Us</a>
                </li>

                <li class="nav-item dropdown">
                <a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
                    Eastern Europe
                </a>
                <div class="dropdown-menu" aria-labelledby="navbarDropdown">
                    <a class="dropdown-item" href="#">Croatia</a>
                    <a class="dropdown-item" href="#">Crezh</a>
                </div>
                </li>
            </ul>

        </div>
    </nav>
</div>
<script src="https://code.jquery.com/jquery-3.2.1.slim.min.js" integrity="sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.9/umd/popper.min.js" integrity="sha384-ApNbgh9B+Y1QKtv3Rn7W3mgPxhU9K/ScQsAP7hUibX39j7fakFPskvXusvfa0b4Q" crossorigin="anonymous"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js" integrity="sha384-JZR6Spejh4U02d8jOt6vLEHfe/JQGiRRSQQxSfFWpi1MquVdAyjUar5+76PVCmYl" crossorigin="anonymous"></script>

尝试将“d-Flex justify-content-center align-items-center”类添加到nav元素中。

对