First, I would check to make sure the desired solution works in the query which supplies the form. If you have a data type error it would be very apparent though a query.
Second, are [QuantityBtB],[QuantityStock], etc the names of your text box objects? If they are, then you may want to try referring to the actual recordsource field name. That is, if you truly want a Sum of all of the records.
So, if [QuantityStock] object is supplied by the query field "[QtyInStock]" then refer to the column headings of your query when doing your formula instead of the combo box name. Just make sure your textbox object is located in the correct area of the form.