How to use get_post_custom function on the blog page?

The basic problem is that there is no variable $post in your header.php. That variable might exist in the global scope, but your code operates in a function scope of load_template() which was called by get_header().

So you have four options:

1. Import the global variable into your function with the global keyword.
   global $post;

   // make sure everything is set up as a post object
   $post   = get_post( $post );
   $values = get_post_custom( $post->ID );
   

2. Use get_queried_object_id() to get the ID, similar to hepii110’s suggestion.

   $values = get_post_custom( get_queried_object_id() );
   

3. Use get_the_ID(). This does almost the same as version 1.

   $values = get_post_custom( get_the_ID() );
   

4. Call get_post_custom() without the post ID. It will try to find the correct ID automagically.

   $values = get_post_custom();