BUG: Can not get inline css code. how to use this?

There is no built-in command to get inlined CSS, but you can find how it was implemented in the newsletter plugin here: https://github.com/artf/grapesjs-preset-newsletter/blob/master/src/openExportCommand.js

The core functionality is a few lines of code by using the juice package

import juice from 'juice';
// ...
const getInlined = (editor) => {
 const html = editor.getHtml();
 const css = editor.getCss();
 return juice(`${html}<style>${css}</style>`, { /* juice options */ });
}

How and where to put that function, is up to you