Can't post large volume of text in Postman because of Bodyparser / Server file error - express

Is there a specific way I should set-up my body-parser to deal with large volumes of Text like articles when I am making a post to Postman? I am getting an entity-parse error when I try to make a post on Postman and the terminal in my IDE flags up the below segments of code that are causing the error. Do I need to set the parser to allow higher volumes of text?

My routes, controller, model etc match to Postgres so I believe the error is somewhere in my server.js file to do with BodyParser.

user.routes.js segment that is being flagged in IDE terminal

**app.use(function (req, res, next) {
        res.header(
            "Access-Control-Allow-headers",
            "x-access-token, Origin, Content-Type, Accept"
        );
        next();
    });**

error message on console

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at ServerResponse.setHeader (_http_outgoing.js:485:11)
    at ServerResponse.header 
...
...backend\server\routes\user.routes.js:7:13
...
...backend\server.js:67:5

server.js

**app.use((err, req, res, next) => {
    res.status(500).json({
        error: err,
        message: 'Internal server error!',
    })
    next()
})**

json posting on Postman

{
    "articleTitle": "Amazon’s Plastic Problem Revealed",
    "articleContent": "Oceana released a report based on an analysis of e-commerce packaging data that found Amazon generated 465 million pounds of plastic ...... want and offer plastic-free packaging as an option at checkout.”,
    "articlePhotos": null,
    "articleAuthor": 6,
    "articleType":  2
}

error message on postman

{
    "error": {
        "expose": true,
        "statusCode": 400,
        "status": 400,
        "body": "{\r\n    \"articleTitle\": \"Amazon’s Plastic Problem Revealed\",\r\n    \"articleContent\": \"Oceana released a report based on an analysis of e-commerce packaging data that found Amazon generated 465 million pounds of plastic packaging waste last year. This is comprised of the .... Listen to its customers: As an immediate measure Amazon should give its customers what they want and offer plastic-free packaging as an option at checkout.”,\r\n    \"articlePhotos\": null,\r\n    \"articleAuthor\": 6,\r\n    \"articleType\":  2\r\n}\r\n",
        "type": "entity.parse.failed"
    },
    "message": "Internal server error!"
}

server.js

const express = require("express");
const bodyParser = require ("body-parser");

// creates an en express app
const app = express();

app.use((req, res, next) => {
    res.header('Access-Control-Allow-Origin', '*');
    res.header(
        'Access-Control-Allow-headers',
        'Origin, X-Requested-With, Content-Type, Accept',
    );
    next();
});

// parse requests of content-type
app.use(bodyParser.json({limit: '50mb'}));

//bp used to process form data as json
app.use(bodyParser.urlencoded({limit: '50mb', extended: true }));

...

try {
    db.sequelize.authenticate();
    console.log('Connection made to the database made.');
    } catch (error) {
    console.error('unable to connect:', error);
}

// db.sequelize.sync();
// force: true will drop the table if it already exists
db.sequelize.sync({ force: false })
    .then(() => {
    console.log(`Drop and re-sync database with { force: true }`)
    //initial()
});

// simple route
app.get('/', (req, res) => {
    res.json({ message: "Connected to the sustainable scuba database." });
});

app.disable('x-powered-by')
app.use(bodyParser.json())
app.use(bodyParser.urlencoded({extended: false}))

app.use((err, req, res, next) => {
    res.status(500).json({
        error: err,
        message: 'Internal server error!',
    })
    next()
})

app.listen(9001, () => {
    console.log('app now listening for requests!!!')
})