Magento2 Product Save – Unable to Save Product

I ran into the same problem when i tried to change data on quote items with the checkout_cart_product_add_after event. This event is fired after every product add, but before the cart is saved. Check the Magento\Checkout\Controller\Cart\Add line 108-113 for reference. So if you want to save the quote item on a new session there is no quote yet and there is no quote_id to reference to.

In my case, i just left the $item->save(); out, because it will be saved afterwards automatically.

Simple solution if you are not planning to use MSI you can simply disable all modules related to it by using below

php bin/magento module:disable -f Magento_Inventory Magento_InventoryAdminUi Magento_InventoryApi Magento_InventoryBundleProduct Magento_InventoryBundleProductAdminUi Magento_InventoryCatalog Magento_InventorySales Magento_InventoryCatalogAdminUi Magento_InventoryCatalogApi Magento_InventoryCatalogSearch Magento_InventoryConfigurableProduct Magento_InventoryConfigurableProductAdminUi Magento_InventoryConfigurableProductIndexer Magento_InventoryConfiguration Magento_InventoryConfigurationApi Magento_InventoryGroupedProduct Magento_InventoryGroupedProductAdminUi Magento_InventoryGroupedProductIndexer Magento_InventoryImportExport Magento_InventoryIndexer Magento_InventoryLowQuantityNotification Magento_InventoryLowQuantityNotificationAdminUi Magento_InventoryLowQuantityNotificationApi Magento_InventoryMultiDimensionalIndexerApi Magento_InventoryProductAlert Magento_InventoryReservations Magento_InventoryReservationsApi Magento_InventoryCache Magento_InventorySalesAdminUi Magento_InventorySalesApi Magento_InventorySalesFrontendUi Magento_InventoryShipping Magento_InventorySourceDeductionApi Magento_InventorySourceSelection Magento_InventorySourceSelectionApi Magento_InventoryShippingAdminUi Magento_InventoryDistanceBasedSourceSelectionAdminUi Magento_InventoryDistanceBasedSourceSelectionApi Magento_InventoryElasticsearch Magento_InventoryExportStockApi Magento_InventoryReservationCli Magento_InventoryExportStock Magento_CatalogInventoryGraphQl Magento_InventorySetupFixtureGenerator

As you are reading this you probably planning to use MSI so here's how you can solve it :),

In MSI inventory_source_item table is responsible for inventory management. Make sure that products whic you migrated to 2.3.x are exists in table.

If not then try to add those manually for a particular SKU like,

Enter in inventory_source_item with your desired qty and with source code.

See below screenshot.

Here I've no qty so data is like 0.0000 you can add your desired qty.

Note : if you've multiple source then add new entry to that table with different source.

Hope it will solve your problem.