Hello,
Rare to find anyone with any real knowledge of RCS simulation in HFSS, so take everything I say with a grain or two of salt.
I suggest starting with HFSS's own ogive-IE example. file--> open examples --> RF_microwave --> Ogive.
It looks like you setup your sweep correctly. However, in RCS simulations and MoM solving you dont need to define a large air box. The simulation assumes open air.
Also, if this is a large structure, perhaps ray optics to speed up the solving time? Check out: **broken link removed** to see how to change to ray optics