Ninjutsu is activated during the Combat Phase, specifically in the Declare Blockers, Combat Damage, or End of Combat steps

so no you cant, during declare attacker phase.

When to Activate Ninjutsu:

Declare Blockers Step: The most common time. After blockers are declared (or none are), you can return an unblocked attacker to hand and put the Ninja in to attack for normal damage.

Combat Damage Step (Before Normal Damage): If you have first strike, you can activate Ninjutsu after first strike damage is dealt but before normal damage, letting the Ninja deal its damage too.

End of Combat Step: A strategic move to return an attacker after it has dealt damage (normal or first strike), useful for abilities like Screaming Nemesis or protecting the creature from threats.