You need to add the quotes around chatbot.

graph_builder.add_node("chatbot", chatbot)
graph_builder.add_node("chat_history",chat_history)
graph_builder.add_node("product_rec_prompt", product_rec_prompt)

graph_builder.add_edge(START, "chatbot") # Made the change here 

graph_builder.add_edge("chat_history", "product_rec_prompt")
graph_builder.add_edge("product_rec_prompt", "chatbot")
graph_builder.add_edge("chatbot", END)
graph = graph_builder.compile()
graph.get_graph().print_ascii()