Had a look at your db. In the query that the subform is based on remove the categoryID field. On the subform, make it an unbound field. You do not need to store this value. If you attempt to store this value, the productID attempts to create a new product (rather than letting you choose an item).
To give you an idea of what is happening before you implement the solution that I have suggested, Change the property of the productcbo combobox on your subform to display the first column.
run the form and select a categoryID and see the value that is entered into the productid (next autonumber). This is the cause of your error.
Hope this works for you.
Originally posted by River
I got an error message when try to select item in second combo, I can't find the reason so I need help from averybody.