One must make use of the different centering methods provided by Bootstrap 4...
Utilize text-center
for inline elements.
Utilize justify-content-center
for flexbox elements (e.g. form-inline)
Additionally, to offset the column, the col-sm-* should be enclosed within a .row, and the .row should be inside a container...
<section id="cover">
<div id="cover-caption">
<div id="container" class="container">
<div class="row">
<div class="col-sm-10 offset-sm-1 text-center">
<h1 class="display-3">Welcome to Bootstrap 4</h1>
<div class="info-form">
<form action="" class="form-inline justify-content-center">
<div class="form-group">
<label class="sr-only">Name</label>
<input type="text" class="form-control" placeholder="Jane Doe">
</div>
<div class="form-group">
<label class="sr-only">Email</label>
<input type="text" class="form-control" placeholder="<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="89e3e8e7eca7ede6ecc9ecf1e8e4f9e5eca7eae6e4">[email protected]</a>">
</div>
<button type="submit" class="btn btn-success ">okay, go!</button>
</form>
</div>
<br>
<a href="#nav-main" class="btn btn-secondary-outline btn-sm" role="button">↓</a>
</div>
</div>
</div>
</div>
</section>