"It
seems that the Decedent's Estate box should be checked..."
Yes, you have to make sure the appropriate box is checked when you start the return; both the type of return (i.e., whether it is an estate or trust as opposed to a business) and then whether the return is for a trust or estate (see screenshots).
If you start a return and designate that it is a trust, there does not appear to be a way to change the return to that of an estate (it does appear to work the other way around, however).
