Sometimes Magento 2.3.6 upgrade after fetching this issue, then run below SQL query inside the direct database to get the solution.

INSERT INTO `eav_attribute` (`entity_type_id`, `attribute_code`, `attribute_model`, `backend_model`, `backend_type`, `backend_table`, `frontend_model`, `frontend_input`, `frontend_label`, `frontend_class`, `source_model`, `is_required`, `is_user_defined`, `default_value`, `is_unique`, `note`) VALUES

(4, 'custom_layout_update_file', NULL, 'Magento\Catalog\Model\Product\Attribute\Backend\LayoutUpdate', 'varchar', NULL, NULL, 'select', 'Custom Layout Update', NULL, 'Magento\Catalog\Model\Product\Attribute\Source\LayoutUpdate', 0, 0, NULL, 0, NULL),
(3, 'custom_layout_update_file', NULL, 'Magento\Catalog\Model\Category\Attribute\Backend\LayoutUpdate', 'varchar', NULL, NULL, 'select', 'Custom Layout Update', NULL, 'Magento\Catalog\Model\Category\Attribute\Source\LayoutUpdate', 0, 0, NULL, 0, NULL);