[Vuejs]-Why all Flow types in my VueJs files ('*.vue') are underlined?

If you’re using PHPStorm, it looks like you should install the Vue.js plugin: https://www.jetbrains.com/help/phpstorm/vue-js.html

Before you start, install and enable the Vue.js plugin. The plugin is not bundled with PhpStorm, but it can be installed from the JetBrains plugin repository …. That is all, no additional configuration is required to have all Vue.js-related features work in any project