Why can't i fly but i Pokemon already have HM 03 fly?

Answer 1

your gym badge is not high enough for you to use fly depending on what game you have you have to earn a cirten gym badge for you to use any HM just beat the next few gyms and you will be there soon

Answer 2

In real life, humans cannot fly because we do not have the physical adaptations required for sustained flight, like wings. In the Pokemon game, the HM 03 Fly allows specific Pokemon to transport you between locations within the game world, but it does not grant the ability to physically fly.