Hello BlueGoblin
In order to get access to Trainers Valley, you need first to fight Red on top of Mt Silver (no matter if you lose against him), and get over 120 Pokémon from Kanto in your Pokedex and more than 34 evolved.
Once you have done these requirement, take subway from Saffron or Goldenrod station, you should get the choice to go on Trainers Valley.
Feel free to tell me if you need further assistance about this quest :Sing:
Thank you so much! :Grin: