Create a missing block

yourmodule.post_update.php

/**
 * Create a missing block (with empty fields but matching UUID).
 */
function yourmodule_post_update_create_a_missing_block() {
  $block_entity_manager = \Drupal::service('entity.manager')->getStorage('block_content');

  $missing_uuid = 'CE7DA6B0-C2A0-4EE9-8F6B-8854A3705FB7';
  $block = $block_entity_manager->create([
    'info' => 'Block title goes here',
    'type' => 'block_type_machine_name_here',
    'uuid' => $missing_uuid,
  ]);
  $block->save();
}