Can’t seem to get [shortcode]s to work

function register_shortcodes(){
    add_shortcode( 'my_shortcode', 'my_shortcode' );
    }
add_action( 'init', 'register_shortcodes'); 
function my_shortcode( $atts ) {
    return "--my_shortcode--";
    }

Adding this code to a plugin I’ve been making (specifically for this) doesn’t do anything. I don’t think the function my_shortcode( $atts ) is even called.

I’ve tried putting “[my_shortcode]” in posts, comments, template(s).php. return "--my_shortcode--"; never executes.

I’ve also put the above piece of code in functions.php which gives me error

Fatal error: Call to undefined function add_shortcode() in C:localhost~wp-includesfunctions.php on line 17

line 17 : add_shortcode( 'my_shortcode', 'my_shortcode' );