问题通常是由于没有正确设置回调URL导致的。您可以在开始云端录制之前运行setVideoMixingLayout方法来设置视频混合布局。请确保在调用startRecordingService方法之前，已设置回调URL并且您的服务器已接受并处理来自Agora的回调请求。以下是示例代码：

func startRecording() {

rtcEngine.setVideoMixingLayout(layout)

let channelName = "your_channel_name"

let profile = AgoraCloudRecordProfileConfig( isAudioOnly: false, channelType: .communication, decryptionMode: .aes128xts, secret: "", recordType: [.audio, .video], appliteDir: FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0].path, recordFileRootDir: "", layoutConfig: layout, maxIdleTime: 30 )

let recordingConfig = AgoraCloudRecordConfig( appId: "your_app_id", channelKey: "your_channel_key", channelName: channelName, uid: "your_uid", isMixingEnabled: true, mixResolution: AgoraVideoDimensions(width: 640, height: 360), fps: AgoraVideoFrameRate.fps15, bitrate: AgoraVideoBitrateStandard, maxResolutionUid: "", transcodingConfig: nil, subscribeVideoUids: nil, subscribeAudioUids: nil, clientRequest: nil )

agoraKit.startCloudRecording(profile, config: recordingConfig)

}

请注意，以上示例代码仅用于说明问题，并且您需要将其自定义以满足您的实际需求。