How to solve "The stock item was unable to be saved"?

Question

I was using the Magento 2.3.2 normally, but since I updated to Magento 2.3.3 I'm not able to create a new product anymore. I'm receiving this message below.

The stock item was unable to be saved. Please try again.

My exception.log is logging this message:

[2019-11-06 17:30:42] main.CRITICAL: The stock item was unable to be saved. Please try again. {"exception":"[object] (Magento\Framework\Exception\CouldNotSaveException(code: 0): The stock item was unable to be saved. Please try again. at /Users/rafael/dev/mystoremagento/vendor/magento/module-catalog-inventory/Model/Stock/StockItemRepository.php:187, Magento\Framework\Exception\CouldNotSaveException(code: 0): Could not save Source Item at /Users/rafael/dev/mystoremagento/vendor/magento/module-inventory/Model/SourceItem/Command/Handler/SourceItemsSaveHandler.php:78, Zend_Db_Statement_Exception(code: 23000): SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (mystoremagento.inventory_source_item, CONSTRAINT INVENTORY_SOURCE_ITEM_SOURCE_CODE_INVENTORY_SOURCE_SOURCE_CODE FOREIGN KEY (source_code) REFERENCES inventory_source (source_code) ), query was: INSERT INTO inventory_source_item (source_code, sku, quantity, status) VALUES (?, ?, ?, ?) ON DUPLICATE KEY UPDATE quantity = VALUES(quantity), status = VALUES(status) at /Users/rafael/dev/mystoremagento/vendor/magento/framework/DB/Statement/Pdo/Mysql.php:110, PDOException(code: 23000): SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (mystoremagento.inventory_source_item, CONSTRAINT INVENTORY_SOURCE_ITEM_SOURCE_CODE_INVENTORY_SOURCE_SOURCE_CODE FOREIGN KEY (source_code) REFERENCES inventory_source (source_code) ) at /Users/rafael/dev/mystoremagento/vendor/magento/framework/DB/Statement/Pdo/Mysql.php:91)"} []

Even doing these tests it doesn't work:

• Disabling all third-party module
• Running the install process (using the same DB)
• Exporting and importing the DB

Answer 1

For anyone facing this issue and MSI removal not being an option, make sure a "Stores->(Inventory) Sources" with source code default exists. There's a hardcoded fallback to it.

Other stuff to try:

• set all your indexers to "On Schedule";
• make sure to have one entry on table cataloginventory_stock with id 1 and the stock name "Default";

Answer 2

I fixed it removing all Magento MSI modules running these commands:

composer require yireo/magento2-replace-inventory:2.3.*
bin/magento s:upgrade
bin/magento deploy:mode:set developer
bin/magento setup:static-content:deploy -f
bin/magento setup:di:compile
bin/magento cache:flush
bin/magento index:reindex