featured image not showing

Currently im making new theme and had the idea of adding featured image in the admin side of wordpress, unfortunately its not working this is what i have tried

I have added this code in functions.php

add_theme_support( ‘post-thumbnails’);
i also tried to change it

add_theme_support( 'post-thumbnails', array( 'post' ) ); // Add it for posts
add_theme_support( 'post-thumbnails', array( 'page' ) ); // Add it for pages

after i refresh and log in to my admin panel and tried to create new post or page featured image is not displaying and

I also tried deactivating my custom theme and activating Twenty Fifteen the featured images shows but if i activate my custom theme again the featured image is gone again

and also i have tried to look at SCREEN OPTIONS but i dont have any availble checkbox related to featured image..Please help me guys

Can someone help or guide me what to do this because im new to this 🙂