You should not use <Link> outside a <Router>

I'm assuming that you are using React-Router V4, as you used the same in the original Sandbox Link.

You are rendering the Main component in the call to ReactDOM.render that renders a Link and Main component is outside of Router, that's why it is throwing the error:

You should not use <Link> outside a <Router>

Changes:

1. Use any one of these Routers, BrowserRouter/HashRouter etc..., because you are using React-Router V4.

2. Router can have only one child, so wrap all the routes in a div or Switch.

3. React-Router V4, doesn't have the concept of nested routes, if you wants to use nested routes then define those routes directly inside that component.

---

Check this working example with each of these changes.

Parent Component:

const App = () => (
  <BrowserRouter>
    <div className="sans-serif">
      <Route path="/" component={Main} />
      <Route path="/view/:postId" component={Single} />
    </div>
  </BrowserRouter>
);

render(<App />, document.getElementById('root'));

Main component from route

import React from 'react';
import { Link } from 'react-router-dom';

export default () => (
  <div>
    <h1>
      <Link to="/">Redux example</Link>
    </h1>
  </div>
)

Etc.

---

Also check this answer: Nested routes with react router v4

For JEST users

if you use Jest for testing and this error happen just wrap your component with <BrowserRouter>

describe('Test suits for MyComponentWithLink', () => {
it('should match with snapshot', () => {
const tree = renderer
  .create(
    <BrowserRouter>
      <MyComponentWithLink/>
    </BrowserRouter>
  )
  .toJSON();
 expect(tree).toMatchSnapshot();
 });
});

Enclose Link component inside BrowserRouter component

 export default () => (
  <div>
   <h1>
      <BrowserRouter>
        <Link to="/">Redux example</Link>
      </BrowserRouter>
    </h1>
  </div>
)