You will get the white meeple after your second game.
If you win the tutorial game, you get the white meeple straight away. Otherwise you need to play once again the tutorial or any training game.
Training games count towards the bot challenge. So you can pick the missing bot from there. Of course online games also count for the bot challence. In short you can beat the bots in training or online, it is the same.
The only limitation is if you are not strong enough in online play. Then your rating does not go up enough to unlock all bots. In that case, you will never have access to all bots unless you spend a few coins to unlock them.
Have fun!