Here's why:
* Indago Pokeblocks were introduced in Pokemon Emerald. Pokemon Ruby came before Emerald, so the game doesn't have the mechanic to create them.
* Indago Pokeblocks have a specific effect: They primarily boost a Pokemon's Special Attack stat.
Instead of Indago Pokeblocks, Pokemon Ruby lets you make other types of Pokeblocks:
* Spicy: Boosts Attack
* Dry: Boosts Speed
* Bitter: Boosts Defense
* Sweet: Boosts Special Defense
* Sour: Boosts HP
You can create these Pokeblocks using the Pokeblock Case, which you receive in the game. You'll need to gather berries and then experiment with different combinations to find the right recipe.
Remember: The Pokeblock recipes are a bit random, so you'll have to try different combinations to get the desired Pokeblock type!