Are you attempting to create the stock code via the "Inventory - New" function?  In normal operation, it should check for the existence of the code long before you would ever see the 1004 error.

The one thought that comes to mind is if you have a damaged in index file from an improper system shutdown. Make sure that you have exclusive access to the database and run the "Options - Batch Processing" function with the "Repair Tables" and the "Verify Database Structure" options checked.

If that does not resolve the issue, we'll arrange to get a copy of the database sent to us to see why the initial check for the product isn't working.