In order to create a financial transaction (sales, purchase, inter tracker transfer) against a stock class, you must first have an account type (sales/purchase) from your chart of accounts assigned to this within the tracker settings.
If you attempt to create a financial transaction against a stock class that doesn't have an account assigned to it, Figured will give you an error - "The selected stock class is not mapped to a Xero account. Please select an account for this stock class".
To remedy this, you'll need to head into your tracker settings which can be found on the main page of your production tracker.
Once there, ensure you have your account(s) mapped up correctly, save changes, and then create the financial transaction again.