As already mentioned, you need to configure tsconfig.json. I would add that the include field should contain auto-imports.d.ts and the exclude field should not contain this file:

{
  //...
  "include": [
    // ...
    "auto-imports.d.ts",
  ],
  "exclude": [
    "node_modules" 
    // auto-imports.d.ts should not be inside exclude field
  ]
  //...
}

Read more about the Auto-Import in Vue: https://medium.com/@alekswebnet/auto-import-in-vue-quick-setup-guide-3004d97e749a