ERROR TypeError: Cannot read property 'append' of undefined

I want to build a column chart but I got the following error:

ERROR TypeError: Cannot read property ‘append’ of undefined

The error happened in line #42.

My code:

export class OutputAmplitudeComponent implements OnInit, OnDestroy {

constructor(@Inject(SETTING_SERVICE) private settingService: SettingService, private cdr: ChangeDetectorRef) {}

ngOnInit() {        
                this.settingService.registerSetting(HarmonicAmpSetting).pipe(takeUntil(this.ngUnsubHarmonicData)).subscribe(setting => {
                    const OutputAmplitudeData = setting.value;

                    for (let x = 1; x < this.numberOfOutput; x++) {
                        if (this.OutputMode === 'Voltage') {
                            if (phaseNum === 1) {
                                this.ampSource.data[x-1].voltage1 = OutputAmplitudeData[x];
                            } else if (phaseNum === 2) {
                                this.ampSource.data[x-1].voltage2 = OutputAmplitudeData[x];
                            } else if (phaseNum === 3) {
                                this.ampSource.data[x-1].voltage3 = OutputAmplitudeData[x]; 
                            }
                        } else if (this.OutputMode === 'Current') {
                            if (phaseNum === 1) {
                                this.ampSource.data[x-1].current1 = OutputAmplitudeData[x];
                            } else if (phaseNum === 2) {
                                this.ampSource.data[x-1].current2 = OutputAmplitudeData[x];
                            } else if (phaseNum === 3) {
                                this.ampSource.data[x-1].current3 = OutputAmplitudeData[x];
                            }
                        }

                    }   
                    this.updateData();
                });
                this.sciChartInit();
            }   

updateData(){
    var phaseNum1 = [];
    var xData = [];
    for (let x = 1; x < this.numberOfOutput; x++) {
        if (this.ampSource.data != null && this.ampSource.data[x-1] != null) {
            phaseNum1[x - 1] = parseFloat(this.ampSource.data[x - 1].voltage1);
            xData[x - 1] = x;
            if (!isNaN(phaseNum1[x - 1])) {
                this.dataSeries1.append(x,x);
            } else {
                console.log("No num");
            }
        }
    }
}       

async sciChartInit() {

    const { wasmContext, sciChartSurface } = await SciChartSurface.create("chart");

    var phaseNum1 = [];
    var xData = [];
    this.dataSeries1 = new XyDataSeries(wasmContext);
    for (let x = 1; x < this.numberOfOutput; x++) {
        this.dataSeries1.append(x, x);
    }

    const xAxis = new NumericAxis(wasmContext);
    sciChartSurface.xAxes.add(xAxis);
    const yAxis = new NumericAxis(wasmContext);
    sciChartSurface.yAxes.add(yAxis);

    const rendSeries1 = new StackedColumnRenderableSeries(wasmContext);
    rendSeries1.fill = "#dc443f";
    rendSeries1.stroke = "green";
    rendSeries1.strokeThickness = 1;
    rendSeries1.dataSeries = this.dataSeries1;
    rendSeries1.stackedGroupId = "one";

    const verticallyStackedColumnCollection = new StackedColumnCollection(wasmContext);
    verticallyStackedColumnCollection.dataPointWidth = 0.5;
    verticallyStackedColumnCollection.add(rendSeries1);
    verticallyStackedColumnCollection.animation = new ScaleAnimation({ duration: 1000, fadeEffect: true });

    sciChartSurface.renderableSeries.add(verticallyStackedColumnCollection);
    sciChartSurface.chartModifiers.add(new ZoomExtentsModifier(), new ZoomPanModifier(), new MouseWheelZoomModifier());
    sciChartSurface.zoomExtents();

    sciChartSurface.chartModifiers.add(
        new LegendModifier({
            placement: ELegendPlacement.TopRight,
            orientation: ELegendOrientation.Horizontal,
            showLegend: true,
            showCheckboxes: true,
            showSeriesMarkers: true
        })
    );
    return { wasmContext, sciChartSurface };
}}

I don’t understand what is undefined and why.

Thank you.