Depends on the theme you are using. If that doesn’t support widget area for posts only then you will have to edit the sidebar that loads into the single.php.
You can have it render conditionally depending on if it is a post. So, in your sidebar file that loads into single.php have something like:
if( is_single() ) {
// code here to show on posts sidebar
}
The code you enter inside the is_single condition could be raw html like:
<div class="widget widget_text">
<h3 class="widget-title">[enter title]</h3>
<div class="textwidget">[enter widget text]</div>
</div>
or you can get fancy and use the proper function to generate a text widget, like so:
$text = array(
'title' => 'Widget Title',
'text' => 'Widget text.',
'filter' => ''
);
$text_args = array(
'before_widget' => '<div class="widget widget_text">',
'after_widget' => '</div>',
'before_title' => '<h3 class="widget-title">',
'after_title' => '</h3>'
);
the_widget( 'WP_Widget_Text', $text, $text_args );
The end result will be pretty much the same.