Registering Carbon Fields at the right hook.

carbon-fields-setup.php

<?php
use Carbon_Fields\Container\Container;
use Carbon_Fields\Field\Field;

add_action('carbon_register_fields', 'my_carbon_fields_setup');
function my_carbon_fields_setup() {
	Container::make('post_meta', 'test1')
		->show_on_post_type('post')
		->add_fields(array(
				Field::make('complex', 'crb_test1')->add_fields(array(
						Field::make('text', 'test1_link_text'),
						Field::make('text', 'test1_link'),
				)),
		));

	Container::make('post_meta', 'test2')
		->show_on_post_type('post')
		->add_fields(array(
				Field::make('complex', 'crb_test2')->add_fields(array(
						Field::make('text', 'test2_link_text'),
						Field::make('text', 'test2_link'),
				)),
		));

	Container::make('post_meta', 'test3')
		->show_on_post_type('manga')
		->add_fields(array(
			Field::make('relationship', 'crb_relationship')
				->set_post_type('post')
		));
}