Why can you not get on the rocket in Pokemon Ruby?

You can actually get on the rocket in Pokemon Ruby! The part you're probably thinking of is the SS Tidal in Lilycove City, which is a ship that Team Aqua and Team Magma are trying to take over. You can't get on the ship at the beginning of the game because it's blocked off by Team Aqua or Team Magma members.

Here's how to get on the ship:

* Complete the story: You'll need to progress through the game, defeating Team Aqua or Team Magma and their leader, before you can get on the SS Tidal.

* Talk to Captain Stern: After defeating the leader, you'll be able to talk to Captain Stern. He'll ask for your help getting the ship back under control.

* Board the ship: Captain Stern will allow you to board the SS Tidal, and you can explore it freely.

If you're still having trouble getting on the ship, please clarify what you're trying to do!