In order to be able to use camera, for example, for taking images through <input type="file" accept="image/*" capture> HTML tag, you need to ask camera permission. If you need to capture video and audio, you need to ask also microphone permission.
To ask permissions, for example, you can use the permission_handler plugin!

Code example:

import 'package:permission_handler/permission_handler.dart';

Future main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Permission.camera.request();
  await Permission.microphone.request(); // if you need microphone permission

  runApp(MyApp());
}

Configure Android

On Android, you need to add these permissions in your AndroidManifest.xml file to be able to use camera for taking images and videos:

<uses-permission android:name="android.permission.INTERNET"/>
<uses-permission android:name="android.permission.CAMERA" />
<uses-permission android:name="android.permission.RECORD_AUDIO" />
<uses-permission android:name="android.permission.MODIFY_AUDIO_SETTINGS" />
<uses-permission android:name="android.permission.VIDEO_CAPTURE" />
<uses-permission android:name="android.permission.AUDIO_CAPTURE" />

Also, you need to add the following code inside the <application> tag of your AndroidManifest.xml file:

<provider
    android:name="com.pichillilorenzo.flutter_inappwebview.InAppWebViewFileProvider"
    android:authorities="${applicationId}.flutter_inappwebview.fileprovider"
    android:exported="false"
    android:grantUriPermissions="true">
    <meta-data
        android:name="android.support.FILE_PROVIDER_PATHS"
        android:resource="@xml/provider_paths" />
</provider>

Configure iOS

On iOS, you need to add the following properties in your Info.plist file to be able to use camera for taking images and videos:

<key>NSMicrophoneUsageDescription</key>
<string>Flutter requires acess to microphone.</string>

<key>NSCameraUsageDescription</key>
<string>Flutter requires acess to camera.</string>

If you open this file In Xcode, then the NSMicrophoneUsageDescription property is represented by Privacy - Microphone Usage Description and NSCameraUsageDescription is represented by Privacy - Camera Usage Description.