You need to restart the game and try again. There is a huge load on the servers as they are adding new countries to the list where Pokemon GO is available. If restarting the game doesn’t work, reinstall it. If that doesn’t work, have patience and try in a few days.
Why is Pokemon go not letting me catch Pokémon?
If the game detects you are changing drastically varying locations too often, you will not be able to catch pokemon, and pokemon immediately run away when you do try to catch them. This is most likely due to GPS spoofing or another cheat of some kind.