Q:

When using browser-sync,why is the browser saying cannot get, while everything seems fine

This is the screenshot of cmd:

I wrote 
browser-sync start --server --files "app\index.html"

A:

Looking at your screenshot it appears that your project directory is situated in the Desktop folder and is structured as follows:
confusion
│
└───app
    │
    └───index.html

In which case you could try instructing Browsersync to start a server from the app directory. If you cd to your folder named confusion then run: 
browser-sync start --server "app" --files "app/index.html"
Note the addition of "app" after the --server flag.

