you have two objectives here that need to be done separately. first the builder, that error is common if you have a space somewhere that you aren't suppose to such as between your fields and the multiplier. it should look like
Code:
=[qryInvoiceTotal]![Quantity]*[qryInvoiceTotal]![UnitPrice]
when you are done if you look at the row source of the text box, no spaces between anything. if that does not fix it then look at this http://access.mvps.org/access/forms/frm0018.htm or google "access #NAME" , there's lots of other people that have had the same issue.
after that is working then go to the properties of the text box and rename it "InvAmt" on the other tab.