why wont the "Post Thumbnail box" display in post panel

I’ve been trying to incorporate thumbnails that link to a larger image in my theme. I’ve come across a handful of tutorials saying

Theme authors, I’m going to make this simple for you. You only need one line of code to turn this feature on for your users. Add this to your theme’s functions.php file:

add_theme_support( ‘post-thumbnails’ );

So i put this code in my functions.php theme file but I don’t see the “post thumbnail box” that is suppose to display.
What am I doing wrong?