HM Fly is actually learned by teaching the move to a Pokémon through the Move Reminder. You can find the Move Reminder in the Pokémon Center, and it allows you to teach HMs to your Pokémon.
Here's how you get HM Fly:
1. Find the Move Reminder: This is located in the Pokémon Center, on the right side of the room, near the Nurse Joy's desk.
2. Find a compatible Pokémon: Not all Pokémon can learn HM Fly. You need a Pokémon that can learn the Flying-type moves.
3. Use the Move Reminder: Talk to the Move Reminder, and select HM Fly from the list of moves. You'll need to have obtained HM Fly from the story.
4. Teach HM Fly: The Move Reminder will ask you to choose a compatible Pokémon to teach HM Fly to.
Important Note: In Pokémon Red Rescue Team, HM Fly is used for navigation within the game's world, allowing you to travel between different areas quickly.