@ Samantha В выпадающей кнопке вы пропустили добавление атрибута data-toggle='dropdown'
.
Это должно быть, как показано ниже,
<main role="main" class="col-md-9 ml-sm-auto col-lg-10 px-4">
<div class="d-flex justify-content-between flex-wrap flex-md-nowrap align-items-center pt-3 pb-2 mb-3 border-bottom">
<h1 class="h2">Dashboard</h1>
<div class="btn-toolbar mb-2 mb-md-0">
<div class="btn-group mr-2">
<button type="button" class="btn btn-sm btn-outline-secondary">Share</button>
<button type="button" class="btn btn-sm btn-outline-secondary">Export</button>
</div>
<div class="dropdown">
<button type="button" class="btn btn-sm btn-outline-secondary dropdown-toggle" data-toggle="dropdown">
<span data-feather="calendar"></span>
This week
</button>
<div class="dropdown-menu" aria-labelledby="dropdownMenuButton">
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>
<a class="dropdown-item" href="#">Something else here</a>
</div>
</div>
</div>
</div>