If error in indexing after upgrade then run below sql query in phpmyadmin
CREATE SQL SECURITY INVOKER VIEW `inventory_stock_1` AS SELECT DISTINCT legacy_stock_status.product_id, legacy_stock_status.website_id, legacy_stock_status.stock_id, legacy_stock_status.qty quantity, legacy_stock_status.stock_status is_salable, product.sku FROM cataloginventory_stock_status` `legacy_stock_status` INNER JOIN `catalog_product_entity` product ON legacy_stock_status.product_id = product.entity_id;
and run below commands
* Clear `var/generation`
* Clear `var/cache`
* Enable Magento modules: `php bin/magento module:enable --all`
* Compile DI `php bin/magento setup:di:compile`