If either target is an illegal target as Domri’s Ambush tries to resolve, the creature you control won’t deal damage.

You can’t cast Domri’s Ambush unless you choose both a creature you control and a creature or planeswalker you don’t control as targets.

If the creature you control is an illegal target as Domri’s Ambush tries to resolve, you won’t put a +1/+1 counter on it. If that creature is a legal target but the other target isn’t, you’ll still put the counter on the creature you control.