I am creating React web app from two folders and the dynamic routes within the app return these errors. Static routes work just fine. I get the errors: Uncaught SyntaxError: Unexpected token '<' manifest.json:1 Manifest: Line: 1, column: 1, Syntax error.

import React, { Component } from 'react';
import './NavBar.css';
import Axios from 'axios';
import { observer } from 'mobx-react';
import UserStore from '../../src/Stores/UserStore';

class NavBar extends Component {
    constructor(props) {
        super(props);
        this.state = {
            results: {},
            apiLoading: false,
            message: ''
        };
    }

    async componentDidMount() {
        try{ //check if the user is logged in
            let res = await fetch('/isLoggedIn', { 
                method: 'post',
                headers: {
                    'Accept': 'application/json',
                    'Content-Type': 'application/json'
                }
            });

            let result = await res.json();

            if (result && result.success) {
                UserStore.loading = false;
                UserStore.isLoggedIn = true;
                UserStore.username = result.username;                
            }else{
                UserStore.loading = false;
                UserStore.isLoggedIn = false; 
            }
        }
        catch(error){
            UserStore.loading = false;
            UserStore.isLoggedIn = false;
        }  
    }

render () {

        if(UserStore.loading) {
            return(

              <div>
                <p>Loading, please wait...</p>
              </div>

            );
        }else{            
            if(UserStore.isLoggedIn){

                let hrefString = '/account/' + UserStore.username;
                return(
                    <div className="navigation">

                        <div className="HomeButton">
                            <a href="/"><h3>Stock Track</h3></a>
                            <i id="#icon" className="fas fa-globe"></i>
                        </div>

                        <div className="NavButtons">
                            <a href={hrefString}>My Account</a>
                            <a href="/lessons">Lessons</a> 
                        </div>

                    </div>
                );

            }else{
                return(
                    <div className="navigation">

                        <div className="HomeButton">
                            <a href="/"><h3>Stock Track</h3></a>
                            <i id="#icon" className="fas fa-globe"></i>
                        </div>

                        <div className="NavButtons">
                            <a href="/register">Register</a>
                            <a href="/login">Login</a>
                            <a href="/lessons">Lessons</a>
                        </div>

                    </div>
                );
            }
        }
}

In the other folder (folder 2) I have this code that effectively joins both folders:

app.get('/*', function(req, res) {
    res.sendFile(path.join(__dirname, 'build', 'index.html'));
});

And this folder contains a router contains:

isLoggedIn(app, db) {

        app.post('/isLoggedIn', (req,res) => {

            if(req.session.userID) {
                let cols = [req.session.userID];

                db.query('SELECT * FROM users WHERE id = ? LIMIT 1', cols, (err, 
                         data, fields) => { //add cols 
                    if(data && data.length ===1){
                        res.json({
                            success: true,
                            username: data[0].username
                        });
                        return true;
                    }else{
                        res.json({
                            success: false
                        })
                    }
                })
            }else{
                res.json({
                    success: false
                });
            } 
        });

    }

In my App.js (folder 1) I have a router that includes:

<Router> 
        <Switch>

          {/* This route doesn't return the error */}

          <Route exact path="/marketoverview" component={marketOverview} /> 

          {/* But this route returns the error */}

          <Route path='/account/:username' component={MyAccount}/>

        </Switch>
</Router>

Thank you for your suggestions