In the demo environment the stock coverage can easily get low, as there is no production. You can create new coverage in MM. Check first the currect stock level. You can see the stock from material master (transaction MM03).
Enter the product, press enter and select the view 'Storage location stock'.
Enter the plant and storage location.
Here you can see that there are currently 299 pieces of this product in the stock.
An easy way to increase the stock is to do a goods receipt without purchase order (transaction MB1C).
Enter dates, movement type 501, plant and storage location.
Press enter to get to the next page. Enter the quantity.
Press enter. On the next page you can see the account, where this transaction is posted. Save the posting.
The accounting document created looks following
Check the storage after the posting and you can see, that the balance has increased.
In real life you should moninitor the use of MM movement type 501, because it provides an easy way to manipulate the balances.