App recedes to background，audioEngine.start()

private var audioEngine = AVAudioEngine() private var inputNode: AVAudioInputNode!

func startAnalyzing() {
    inputNode = audioEngine.inputNode
    let recordingFormat = inputNode.outputFormat(forBus: 0)
    let hardwareSampleRate = recordingSession.sampleRate
    inputNode.removeTap(onBus: 0)
    if recordingFormat.sampleRate != hardwareSampleRate {
        print("。")
        let newFormat = AVAudioFormat(commonFormat: recordingFormat.commonFormat,
                                      sampleRate: hardwareSampleRate,
                                      channels: recordingFormat.channelCount,
                                      interleaved: recordingFormat.isInterleaved)
        inputNode.installTap(onBus: 0, bufferSize: 1024, format: newFormat) { buffer, time in
            self.processAudioBuffer(buffer, time: time)
        }
    } else {
        inputNode.installTap(onBus: 0, bufferSize: 1024, format: recordingFormat) { buffer, time in
            self.processAudioBuffer(buffer, time: time)
        }
    }

    do {
        audioEngine.prepare()
        try audioEngine.start()
    } catch {
        print(": \(error)")
    }
}

I back the app to the background and then call startAnalyzing(), which reports an error and the background recording permissions are configured。

error： [10429:570139] [aurioc] AURemoteIO.cpp:1668 AUIOClient_StartIO failed (561145187) [10429:570139] [avae] AVAEInternal.h:109 [AVAudioEngineGraph.mm:1545:Start: (err = PerformCommand(*ioNode, kAUStartIO, NULL, 0)): error 561145187 Audio engine couldn't start.

Is background boot not allowed？