Here's why:
* The Members Card is an item given to you by a specific NPC. You cannot obtain it through any other means within the game's intended design.
* Action Replay is a cheating device. It allows you to modify game data, including giving yourself items you haven't earned.
If you want to play the game legitimately, you won't be able to get the Members Card without using a cheat device.
However, the Members Card is not essential for completing the game or enjoying its content. It primarily unlocks access to the Underground's "Secret Base" feature, which allows you to build a personalized base and interact with other players' bases.
If you're playing Pokemon Diamond, you can have a lot of fun without the Members Card. Focus on exploring the world, catching Pokemon, battling trainers, and completing the main story.