WP_REST_Posts_Controller::create_item_permissions_check PHP Method

    public function create_item_permissions_check($request)
    {
        $post_type = get_post_type_object($this->post_type);
        if (!empty($request['author']) && get_current_user_id() !== $request['author'] && !current_user_can($post_type->cap->edit_others_posts)) {
            return new WP_Error('rest_cannot_edit_others', __('Sorry, you are not allowed to create posts as this user.'), array('status' => rest_authorization_required_code()));
        }
        if (!empty($request['sticky']) && !current_user_can($post_type->cap->edit_others_posts)) {
            return new WP_Error('rest_cannot_assign_sticky', __('Sorry, you are not allowed to make posts sticky.'), array('status' => rest_authorization_required_code()));
        }
        if (!current_user_can($post_type->cap->create_posts)) {
            return new WP_Error('rest_cannot_create', __('Sorry, you are not allowed to create posts as this user.'), array('status' => rest_authorization_required_code()));
        }
        if (!$this->check_assign_terms_permission($request)) {
            return new WP_Error('rest_cannot_assign_term', __('Sorry, you are not allowed to assign the provided terms.'), array('status' => rest_authorization_required_code()));
        }
        return true;
    }

 /**
  * Checks if a given request has access to create an attachment.
  *
  * @since 4.7.0
  * @access public
  *
  * @param WP_REST_Request $request Full details about the request.
  * @return WP_Error|true Boolean true if the attachment may be created, or a WP_Error if not.
  */
 public function create_item_permissions_check($request)
 {
     $ret = parent::create_item_permissions_check($request);
     if (!$ret || is_wp_error($ret)) {
         return $ret;
     }
     if (!current_user_can('upload_files')) {
         return new WP_Error('rest_cannot_create', __('Sorry, you are not allowed to upload media on this site.'), array('status' => 400));
     }
     // Attaching media to a post requires ability to edit said post.
     if (!empty($request['post'])) {
         $parent = get_post((int) $request['post']);
         $post_parent_type = get_post_type_object($parent->post_type);
         if (!current_user_can($post_parent_type->cap->edit_post, $request['post'])) {
             return new WP_Error('rest_cannot_edit', __('Sorry, you are not allowed to upload media to this post.'), array('status' => rest_authorization_required_code()));
         }
     }
     return true;
 }