getting this error when using vllm

for idx, node in enumerate(nodes):
    train_dataset_generator = DatasetGenerator(
        [node],
        llm=llm2,
        question_gen_query=question_gen_query,
        show_progress=True,

    )
    node_questions_0 = train_dataset_generator.generate_questions_from_nodes(num=2)
    for question in tqdm(node_questions_0):  
        prompt = PromptTemplate(question)
        print(f"[Node {idx}] Generated questions:\n {node_questions_0}")
        output = llm2.structured_predict(output_cls=Document, prompt=prompt)
        print(question)
o a JSON string
        json_output = json.dumps(jsonable_encoder(output), ensure_ascii=False)
        
        # Open the file in append mode to add the new entry
        with open('output.json', 'a', encoding='utf-8') as f:
            if not first_entry:
                f.write(',')  # Add a comma before the next entry if it's not the first one
            else:
                first_entry = False  # Update the flag after the first entry
            f.write(json_output)  # Write the JSON string

# After all entries are written, close the list with a ']'
with open('output.json', 'a', encoding='utf-8') as f:
    f.write(']')
Unless there's a more efficent way