You choose a target as Alibou's triggered ability is put on the stack. If that target isn't legal as the triggered ability tries to resolve, it doesn't resolve and you don't scry.

If Alibou leaves the battlefield, artifact creatures you control lose haste. If they haven't been under your control since the turn began and don't otherwise have haste, they can't attack that turn. If they've already attacked, losing haste won't remove them from combat and they'll remain attackers.

If you control no tapped artifacts when the triggered ability resolves (perhaps because they were destroyed or had vigilance), no damage will be dealt and you won't scry.