Please change all indexer as Update by Schedule mode in system-> index management. It's working for me.

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

go to Magento 2.4.2 installation location

cd /var/www/html

Flush cache

sudo php bin/magento cache:flush

Chek Indexer Status

php bin/magento indexer:status

Reset Indexer

php bin/magento indexer:reset

Reindex Indexer

php bin/magento indexer:reindex

Chek Indexer Status

php bin/magento indexer:status

Chek indexer Info

php bin/magento indexer:info

Finally Flush Cache

php bin/magento cache:flush

Restart sudo reboot

Login to admin and Check

I have fixed it. There was an entry missing in cataloginventory_stock

try to run this query

INSERT INTO cataloginventory_stock (stock_id, website_id, stock_name) VALUES ('1', '1', 'default');

If you are using Elasticsearch instead of Mysql. then please check once that elasticsearch is working or not.

sudo service elasticsearch status    // for checking the Running Status
sudo service elasticsearch start    // for Start
sudo service elasticsearch restart    // for Restart