Transitions in ductwork are inserted automatically when the size of the ductwork changes. This typically happens at branches.
If you need a transition in the middle of a straight section of duct, use the Break Duct command to insert a break in the duct.
Ribbon: DM HVAC->Ducts-> Break Duct
Pulldown Menu: DM HVAC->Duct Centerlines->Break Duct
Change the size of the ductwork on one side of the break. When you redraw the ductwork, the transition between the two sizes will be inserted automatically.
If you’re inserting an eccentric transition, see this article for more information.