What you miss is that in your code snippet above you failed to name your own parent style.
In the code, you posted in the 4th line from above need customization according to the name of your actual parent style. So as an example this line:
$parent_style="parent-style"; // This is 'twentyfifteen-style' for the Twenty Fifteen theme.
should look like this in case of using Divi:
$parent_style="divi-style"; // This is 'divi-style' for the Divi theme.
To find out what you should put there instead of parent-style in your case, go to your parent theme folder and open the original (or now parent) functions.php, and there search for wp_enqueue_style.
You will find something like this:
wp_enqueue_style( 'divi-style', get_stylesheet_uri(), array(), $theme_version );
This is in my case where I use Divi theme. Look at the first attribute of the function. That’s what you need.
The other example: In case Twenty Fifteen the first attribute will be twentyfifteen-style.
So once you found the used tag-name for your parent style update your 4th line accordingly.