check_age is not a node, it should be the router from greet to other three nodes, below is my code:

# https://stackoverflow.com/questions/79702608/why-the-condition-in-the-langraph-is-not-working

from IPython.display import display, Image
from langgraph.graph import StateGraph, START, END
from typing import TypedDict, Literal



class PersonDetails(TypedDict):
    name: str
    age: int


def greet(state: PersonDetails):
    print(f"Hello {state["name"]}")
    return state

# Add Literal here to indicates where the router should go
def check_age(state: PersonDetails) -> Literal["can_drink", "can_drive", "minor"]:
    age = state["age"]
    if age >= 21:
        return "can_drink"
    elif age >= 16:
        return "can_drive"
    else:
        return "minor"


def can_drink(state: PersonDetails):
    print("You can legally drink 🍺")
    return state


def can_drive(state: PersonDetails):
    print("You can drive 🚗")
    return state


def minor(state: PersonDetails):
    print("You're a minor 🚫")
    return state


graph = StateGraph(PersonDetails)
graph.add_node("greet", greet)
graph.add_node("can_drink", can_drink)
graph.add_node("can_drive", can_drive)
graph.add_node("minor", minor)

graph.add_edge(START, "greet")
graph.add_conditional_edges(
    "greet",
    check_age,
    {
        "can_drink": "can_drink",
        "can_drive": "can_drive",
        "minor": "minor"
    }
)

graph.add_edge("can_drink", END)
graph.add_edge("can_drive", END)
graph.add_edge("minor", END)

app = graph.compile()

# can should the whole graph in xx.ipynb notebook
display(Image(app.get_graph().draw_mermaid_png()))