Get ACF value after update user using save_post

Your code is working but its not going to print to the screen anything because save_post running in ajax. You can debug the action with the error_log().

First in the wp-config.php you need to turn debug and set it to log into file instead of display

define('WP_DEBUG', true);
define('WP_DEBUG_DISPLAY', false);
define('WP_DEBUG_LOG', true);

It will create you a file in the folder wp-content in the name debug.log

And then you can use error_log() to debug like this:

function get_acf_value ($post_id) {
    $v = get_field('field_5b1d13fce338d', $post_id);
    error_log($v);
    // Incase it is array you can use print_r
    error_log( print_r( $v, true ) );
}
add_action( 'acf/save_post', 'get_acf_value' );