It is not possible to build a practical steam engine in the Roman era because metalurgy had not progressed to the point that sufficient pressure could be maintained to be useful. Even in our era the early steamships had very limited range and steam was superior to sail only in a dead calm.
You could probably build them for some purposes, but it's unlikely any of them would be economical compared to human or animal power.
Ah, thanks. How much would/could metalurgy have to be pushed forward for earlier steam engines?