Magento2 – How to Fix Unable to Save Stock Item Issue

At a quick glance, you're probably running into one or more missing classes. In production mode, Magento doesn't generate any files; instead, it expects you to manually run bin/magento setup:di:compile and bin/magento setup:static-content:deploy on your own.

I see you run bin/magento setup:static-content:deploy; however, you never compile the dependency injection code. This generates all the interceptors, proxies, and other "missing" classes that are normally generated on-the-fly when in developer mode.

I would look in your Apache error logs to see the caught 500 error. You could also check the PHP error logs, as well as the exception and system logs in Magento.

When building a deployment for my site, I do the following steps:

1. Update branch to be deployed: git fetch && git pull
2. Set the deployment mode to production (just to be sure): bin/magento deploy:mode:set production
3. Install composer dependencies: composer install --no-dev --no-suggest --optimize-autoloader
4. Run upgrade to bring database up-to-date: bin/magento setup:upgrade
5. Run Magento compilation for dependency injection files: bin/magento setup:di:compile
6. Generate static content: bin/magento setup:static-content:deploy <lang_Code(s)>

At this point, you should have a bunch of files in the generated, var/view_preprocessed and pub/static folders. The files that you are most likely missing are those in the generated folder.

I have encountered this. To get around it, I used this solution: https://magento.stackexchange.com/a/198109/69191

Other solutions are resetting permissions and flushing your Magento caches, including things like memcache and Redis.