Electing to take 50% bonus depreciation versus 100% bonus depreciation for a heavy truck > 6000 lbs

I am trying to take 50% special depreciation for a heavy truck (> 6,000 lbs) placed in service during 2019 but Turbo tax is not allowing me to do that. It is making me take either 100% or nothing- does anyone know why I might be having this problem?