Wrap the Button in div with "text-center" class.
Just change this:
<!-- wrong -->
<div class="col-md-4 center-block">
<button id="singlebutton" name="singlebutton" class="btn btn-primary center-block">Next Step!</button>
</div>
To this:
<!-- correct -->
<div class="col-md-4 text-center">
<button id="singlebutton" name="singlebutton" class="btn btn-primary">Next Step!</button>
</div>
Edit
As of BootstrapV4, center-block
was dropped #19102 in favor of m-*-auto
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