Dr Vu experiments. Level 18 Robot 3 not working

Dr Vu doesn't allow me to unleash Robot 3, it keeps saying to me.... "Not enough buildings to destroy." All my buildings are repaired and I'm allowed to unleash other disasters except level 3 Robot. It worked once but now that message always appear. Any ideas why? P.s level 18 Dr Vu cost 280k. I'm level 20 with 560k pop.