Check whether Storage Locations has been turned on in Inventory > Configuration > Settings If you use this feature, you need to specify the location where the inventory adjustment is to be made. 

If you have this problem only with a specific product, it's very likely, that the Inventory Location of this product isn't set.

You only see this field with the Developer mode activated (Settings application > General Settings (in the left sidebar) > scroll down > Activate the developer mode).

Then go to the Inventory application > Products > Products > search and open the particular product > go to the Inventory tab > here you can see the Inventory Location setting. Make sure that an inventory location is set.

This should fix the problem and you should be able to adjust the stock of this product.

The problem is your Technical > Company Property > property_stock_inventory doesn't exist for some reason. Did you play hard with the defaults recently (just like me, huh?) lol Just restore it back.

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