用 fetch 请求时错误

错误信息如下，求解答

```
Fetch API cannot load http://127.0.0.1:8001/api/app/5715e829a14a74d2227b98f6. Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost:3000' is therefore not allowed access. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.
```

Start by reproducing the fetch request from http://localhost:3000 and inspect the preflight response from http://127.0.0.1:8001/api/app/5715e829a14a74d2227b98f6. The issue provides no source file, test, or proxy configuration; done would require identifying the missing CORS handling and verifying that the request succeeds.

Written by the indexing model from the issue text.