Looks like there is another Pokémon in another tab. You can catch only one Pokémon in same time. And only in last of opened tabs.
Oh, because the forums should only ever be open on one tab. This prevents a user from having a hundred tabs open or something to catch Pokémon, which lags the site. Check your browser and other devices and see if you have the site open on those, and then close them if you do.
