Receiving and Sending JSON from node.js server to html

I am been working on setting up a server in node.js and creating a website that can send and receive information from the server. This is my code

// server.js
const express = require('express');
const bodyParser = require('body-parser');
const cors = require('cors');  // Import the cors package
const { GoogleGenerativeAI } = require('@google/generative-ai');

const app = express();
const port = 3000;

const apiKey = 'MYAPIKEYTHATISCORRECT';
const genAI = new GoogleGenerativeAI(apiKey);

app.use(cors());
app.use(bodyParser.json());

app.post('/chat', async (req, res) => {
    const { past_conversations } = req.body;

    // Validate past_conversations structure
    if (!Array.isArray(past_conversations) || !past_conversations.every(
        convo => convo.role && convo.parts && Array.isArray(convo.parts) && convo.parts.every(part => part.text)
    )) {
        return res.status(400).json({ error: 'Invalid conversation structure' });
    }

    const userInputValue = past_conversations[past_conversations.length - 1].parts[0].text;

    try {
        const model = genAI.getGenerativeModel({
            model: "gemini-1.5-pro",
            systemInstruction: `Output the feelling for the sentence given, like happy, sad, fear or angry`,
        });

        const generationConfig = {
            temperature: 0,
            topP: 1,
            topK: 128,
            maxOutputTokens: 256,
            responseMimeType: "text/plain",
        };

        const chatSession = model.startChat({
            generationConfig,
            history: past_conversations
        });

        const result = await chatSession.sendMessage(userInputValue);
        const aiResponse = await result.response.text();

        res.json({ output: aiResponse });
    } catch (error) {
        console.error('Error during chat processing:', error);
        res.status(500).json({ error: 'Internal Server Error', details: error.message });
    }
});

app.listen(port, () => {
    console.log(`Server running at http://localhost:${port}/`);
});

// script.js
async function sendMessage() {
        const userInputValue = userInput.value;
        if (!userInputValue) return;
    
        const chatDiv = document.getElementById('chat');
        const userMessageDiv = document.createElement('div');
        userMessageDiv.classList.add('message', 'user');
        userMessageDiv.textContent = userInputValue;
        chatDiv.appendChild(userMessageDiv);
        chatDiv.scrollTop = chatDiv.scrollHeight;
    
        userInput.value = '';
    
        // Push user message to pastConversations
        pastConversations.push({ role: "user", parts: [{ text: userInputValue }] });
    
        try {
            const response = await fetch('http://localhost:3000/chat', {
                method: 'POST',
                headers: {
                    'Content-Type': 'application/json'
                },
                body: JSON.stringify({ past_conversations: pastConversations })
            });
    
            const data = await response.json();
    
            if (!response.ok) {
                throw new Error(data.error || 'Failed to get response from server');
            }
    
            const aiResponse = data.output;
    
            const aiMessageDiv = document.createElement('div');
            aiMessageDiv.classList.add('message', 'ai');
            aiMessageDiv.textContent = aiResponse;
            chatDiv.appendChild(aiMessageDiv);
            chatDiv.scrollTop = chatDiv.scrollHeight;
    
            // Push AI response to pastConversations
            pastConversations.push({ role: "model", parts: [{ text: aiResponse }] });
    
            aitext.push(aiResponse);
            speak(aiResponse);
        } catch (error) {
            console.error('Error:', error);
        }
    }

After the function sendMessage() is called, I got the error 500 Internal Server Error and SyntaxError: Unexpected token ‘T’, “TypeError:”… is not valid JSON. What should I do to fix it? ChatGPT didn’t give any useful suggestions.