You don't choose a target for General Traag's last ability at the time it triggers. Rather, a second "reflexive" ability triggers when you sacrifice another artifact this way. You choose a target for that ability as it goes on the stack. Each player may respond to this triggered ability as normal.
Trample
When General Traag enters, you may sacrifice another artifact. When you do, General Traag deals 4 damage to target creature.
