The tokens enter attacking, even if the attack couldn't legally be declared.

If Nacatl War-Pride becomes a copy of something else after the ability triggers but before it resolves, the tokens will enter as copies of whatever the ex-Nacatl War-Pride is now. If Nacatl War-Pride became a copy of something else and then left the battlefield, the tokens will enter as copies of whatever it was copying when it last existed on the battlefield.

Putting an attacking creature onto the battlefield doesn't trigger "When this creature attacks" abilities. It also won't check attacking restrictions, costs, or requirements.

If Nacatl War-Pride has left the battlefield by the time the triggered ability resolves, the tokens will still enter as copies of Nacatl War-Pride.

In a multiplayer game in which you're allowed to attack only one player each combat, the new creatures will be attacking the same player as Nacatl War-Pride. In a multiplayer game in which you may attack multiple players, you choose the player each new creature is attacking. It must be a player you're allowed to attack.

If it's impossible for Nacatl War-Pride to be blocked by exactly one creature, then the defending player may block it with multiple creatures or may leave it unblocked.