DCIM可能是媒体或照片的一部分
所以如果有效话试试这个，我通常在我的清单中包含

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
 // try to add this 2 lines
<uses-permission android:name="android.permission.READ_MEDIA_IMAGES"/>
<uses-permission android:name="android.permission.READ_MEDIA_VIDEO"/>
 // Optional to add this 
 <uses-permission android:name="android.permission.ACCESS_MEDIA_LOCATION"/>

把我的gradle设置成这样

android {
     compileSdkVersion 33
     ....
     // from deafult config 
     defaultConfig{
      ...
      mindSdkVersion 28
      targetSdkVersion 33

  }

然后对于许可，您可以将它们放在列表中，以防请求时出现

// You can either call this in initState or any listener you have
 // as long ass it can be triggered first on page
 static Future<void> requestStorageorPhotos() async{
    await [
       Permission.photos,
       Permission.storage,
     ].request();
    // try to listen from its changes permission
    if((await Permission.photos.status.isGranted) || (await 
      Permission.storage.status.isGranted)){
      // Do your thing if allow
    }else{
     // or anything in here from not allowing it.
    }
  }

如果您的目标是Android 11及更高版本（targetSdkVersion 30），则需要在AndroidManifest.xml中拥有以下权限才能进行修改和文档访问。

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"
    android:maxSdkVersion="28" />
<uses-permission android:name="android.permission.MANAGE_EXTERNAL_STORAGE" />

您可以在这里找到更多细节https://stackoverflow.com/a/66366102/19292778