Trying to Add a LEFT Sidebar?

I added after

get_header(); ?>
      <aside id="secondary" class="left-sidebar widget-area" role="complementary">
          <?php dynamic_sidebar( 'sleft' ); ?>
      </aside><!-- .sidebar .widget-area -->

Then

aside#secondary {
    width: 20%;
    float: left;
    margin-right: 15px;
}

Now… if you’re going to have both sidebars, you’ll have to adjust the

@media screen and (min-width: 61.5625em)
body:not(.search-results) article:not(.type-page) .entry-content {
    float: left;
    width: 50%;
}

or something like that. You can customize the CSS as you need, but this should put you on the right track.

Also.. FWIW, I created a child theme, and added this to it, rather than modifying the twentysixteen theme directly. This is the recommended method.