d-flex类将覆盖navbar-collapse行为。如果要反转flex方向，请在nav元素上使用它...

<nav class="navbar navbar-expand-lg navbar-light bg-light">
    <div class="container">
        <a class="navbar-brand" href="/"><i class="fas fa-blog"></i> Tech Info</a>
        <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent1" aria-controls="navbarSupportedContent1" aria-expanded="false" aria-label="Toggle navigation">
            <span class="navbar-toggler-icon"></span>
        </button>
        <div class="collapse navbar-collapse" id="navbarSupportedContent1">
            <ul class="nav justify-content-end flex-row-reverse">
                <li class="nav-item">
                    <a class="nav-link my-2 my-sm-0" href="/login">Sing In</a>
                </li>
            </ul>
        </div>
    </div>
</nav>

演示：https://www.codeply.com/p/MgNaErmlWm

按钮的目标必须等于要折叠的div id，然后从div.navbar-collapse中删除d-flex

<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent1"
  aria-controls="navbarSupportedContent1" aria-expanded="false" aria-label="Toggle navigation">
  ...
</button>
<div class="collapse navbar-collapse" id="navbarSupportedContent1">
  ...
</div>

navbar通常不应为容器。尝试取出div class =“ container”。