Does the Pokemon with pick up ability have to enter in battle order work?

The Pokémon with the Pickup ability does not need to enter the battle in order to find items.

Here's how Pickup works:

* At the start of each encounter: The Pokémon with Pickup has a chance to find an item.

* Chance of finding an item: The chance of finding an item depends on the Pokémon's level and the game generation.

* Types of items: The types of items found vary and can include Potions, Revives, Poké Balls, and even rare items like Rare Candies.

Important Note: The Pickup ability only works in wild encounters, not against other trainers.

So, whether or not your Pickup Pokémon is in your battle party is irrelevant. As long as it's in your team, it has a chance to find an item at the start of each wild encounter.