In the Celadon City, when you reach the Team Rocket Hideout in the basement. There’s a locked Elevator the requires a key, when you tried to open the Elevator it keep saying “It doesn’t appear to work without a key…”.
The key is somewhere down in the basement. To go in there, take the stairs on the upper corner, it will lead you down to the next floor.
When you reach this floor, there will be some puzzle. Complete it until you reach the stairs down to the basement.
In the basement, there will be Team Rocket member on the left top corner. He is the one who hold the key for the elevator.
You’ll be fighting this Team Rocket member, when you defeated him, he will throw the key on the ceiling and you need to figure how to get it.
Use the chair in the corner, you can use this as a step stool.
Your Pikachu can now enter the vent, and has a way now to get the Key.
What you only need to do is to reach the key on the ceiling.
That’s it, you can now go back to the Elevator and use it to reach the room of the Team Rocket Boss.
You may also watch our walk though video guide.