I need use is_single in functions.php

is_single most likely only works inside of The Loop, and functions.php loads before you’re in The Loop.

You could try something like this:

add_action( 'wp_head', function(){
    var_dump(is_single());
});

And now you see that is_single() returns true.

EDIT: I was probably off for suggesting it only works inside The Loop, but there’s definitely better places for is_single than in functions.php, and if you’re checking the “singleness” of a page, wait until you’re in the request proper, or the template, to use it.