is_page() function doesnt working

The init hook is too early — WordPress hasn’t yet identified the queried object (post, category, etc.); hence the is_page() doesn’t work there:

add_action('init', 'register_script');

So for conditional tags/functions like is_page(), is_single(), etc. to work (i.e. return the proper result), you should use wp or a later hook:

add_action('wp', 'whatever_function');

But for registering/enqueueing scripts and styles, you should always use the wp_enqueue_scripts hook; so you would use this (and not the above):

add_action('wp_enqueue_scripts', 'register_script');

PS: Credits to @huraji for his helpful comment.