You need to use the various Bootstrap 4 centering methods...
Use text-center
for inline elements.
Use justify-content-center
for flexbox elements (ie; form-inline)
https://codeply.com/go/Am5LvvjTxC
Also, to offset the column, the col-sm-* must be contained within a .row, and the .row must be in 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="jane.doe@example.com">
</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>