import { check, validationResult } from 'express-validator/check';

const validatorSignup = [
    check('user_name').exists().isLength({ min: 4, max: 20 }),
    ....
];

router.post('/signup', (req, res) => {
    try {
        upload(req, res, validatorSignup, (err) => {
            console.log(req.body)
            const errors = validationResult(req.body);
            if (!errors.isEmpty()) {
                return res.status(422).json({
                    err: errors.array()
                });
            }
      ....
    } catch (err) {
        return res.status(500).json({
            err: err
        });
    }
});

onFinished(req, function () { next(err) })
                                    ^
TypeError: next is not a function

router.post('/signup', (req, res) => {
    try {
        upload(req, res, (err) => {
            console.log(req.body)
            const errors = validationResult(req.body);
            if (!errors.isEmpty()) {
                return res.status(422).json({
                    err: errors.array()
                });
            }
      ....
    } catch (err) {
        return res.status(500).json({
            err: err
        });
    }
}, validatorSignup);