The same thing happened in my challenge with Seedot. As others mentioned, it's because you don't have the National Dex. So what I did was use a cheat to get the National Dex. But it's recommended you do this only after getting the Tea from the old woman in Celadon, or you can't get into Saffron without a cheat or something. You could also try what NecrumWarrior suggested too, since finding the National Dex cheat was kinda annoying, at least for me.