If an item isn't showing up in Stock > Products then it has not been made into a product yet and therefore doesn't have a stock code.
Stock products are not automatically generated when a new catalogue is imported, due to the volume of codes of which a majority will not be purchased.
If you wish to create a stock code from a catalogue item, you can do so directly from the catalogue item, alternatively, when adding the line to the sales order you will see the option to ‘Make Product’ against the line and in the requirements.
To create a stock code for an item search for the item in the catalogue.
Publications > Catalogues > (Correct Catalogue for Product) > Search for the item code >Open catalogue item > Make Product
