Full error message. 完整的错误提示.

[ts]
Argument of type 'App' is not assignable to parameter of type 'ReactElement<any>[]'.
  Property 'length' is missing in type 'App'.
[ts] Type 'RegExp' cannot be converted to type 'App'.
[ts]
Type 'RegExp' cannot be converted to type 'App'.
  Property 'render' is missing in type 'RegExp'.

Sample code. 示例代码：

import * as React from 'react';
import * as ReactDOM from 'react-dom';
import App from './lib/app/App';

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

That's because you use <App /> in a ts file, rename it to tsx.
因为你在ts文件中用了<App />，把文件改成tsx*问题就没了。