To be able to enter the inventory we will go to:

Finance → Stock Control → Products

Once we see the list of all our articles we give the "Edit" button of the product that we want to enter the inventory.

Once inside, click on Stock, in the menu on the left and later on the button "Set Stock" as we see in the image below.

The following window will open where we must select the Store to which it belongs and the type of movement we want to do if we regularize the stock or dispose the product, as well as the amount to establish we choose.