Allowing negative stock balances is depend on how you company operates, you may need to allow negative values for some materials. this is specific for material which need to production order prior to a goods receipt, to allowed negative values you need to set up in two location configuration, first change the configuration to allow negative stock at the valuation, plant and storage location level and second , change the material to allow negative stock.
You can use OMJ1 SAP Transaction code or follow this SAP Menu path.
IMG->Material Management->Inventory Management and Physical Inventory->Goods Issue/Transfer Posting->Allow Negative Stocks
In the initial screen you need to select the valuation area where you want to allow negative stock and activate flag then you need to go to the plant level.
After you select Valuation area, go to plant level and the new screen will be displayed. in this screen you can choose what kind of stock type that allowed negative stock.
After you finish set up the configuration in this screen, you can continue to Storage Location level. double click on node Storage Location on the left panel and the new screen will be appeared. In this screen you can select which of storage location allowed negative values.
After you have finished setting up configuration on valuation area, plant and storage location, the next step is you need to set up material master record using MM02 transaction code.
On the initial screen , enter material number and select PLANT DATA/STORAGE 2 screen view and check the field NEG.STOCKS IN PLANT.