I gain Death Cart with Watchtower in hand. I see only options for Topdeck, or trash, but not no reaction. For the ruins that come with Death Cart, though, Watchtower works correctly and I see the "done reacting" option.
Game #25806 on tokyo.
I believe in that case you can continue without using Watchtower by clicking "gain ruins" or something similar below the log, though I agree this isn't ideal if it is the case