Has HTML webpack plugin been installed, or error: cannot find module ‘HTML webpack plugin’

Before, the project directory was:

it ran successfully. Later, a new build file directory was created and the webpack.config.js It is moved to the build file,

package.json Script in

"scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
    "start": "node ./src/koa.js",
    "build": "webpack --config ./build/webpack.config.js"

After configuration, run the

$ npm run build

Results error prompt:

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