Attic truss between rooms

[chiefnerd]

Home Designer Professional 2019:

I'm trying to build attic trusses over the 3-car garage between two 90 degree angled gabled rooms (see plan). I'm new to the program and have successfully built an attic room using a single room, but the auto build feature seems to do the whole house and screws up everything else (please confirm if that's what it does; I cannot find a way to use the build tool just for a particular room). I've built my knee walls in the attic but the tool seems to only give the "Cannot product an attic truss. A flat ceiling must be present for the middle of the truss and floor platforms present for the ends of the truss". I'm hoping this program will be able to do the job but I haven't found any examples of this (just single room examples) or have missed the secret sauce to get this to work. I currently have a truss on top of the garage. Disregard the rest of the house; it's still a work-in-progress. Q1: Can it be done? Q2: How to do it

Thanks.

attic_truss.zip