Time series chart Error generating chart: No features contain non-null values of "system:time_start"

I've calculated NDVI from Sentinel 2 images but when I try to get the chart of the series this message is generated - Error generating chart: No features contain non-null values of "system:time_start". The code is the following:

var s2 = ee.ImageCollection('COPERNICUS/S2')
                  .filterDate('2019-01-01', '2019-12-31')
                  .filter(ee.Filter.lt('CLOUDY_PIXEL_PERCENTAGE', 20))
                  .filterBounds(studyArea);
      
function getNDVI (image) {
    var ndvi = image.normalizedDifference(['B8','B4']).rename("ndvi");
    image = image.addBands(ndvi);
    return image;
}
s2 = s2.median();
s2 = getNDVI(s2).clip(studyArea);
var s2NDVI = s2.select('ndvi');
print(s2NDVI);

var NDVIseries = ui.Chart.image.seriesByRegion(
    s2NDVI, studyArea, ee.Reducer.mean(), 'ndvi', 200,  'system:time_start', 'label')
        .setChartType('ScatterChart')
        .setOptions({
          title: 'NDVI 2016',
          vAxis: {title: '-1+1'},
          lineWidth: 1,
          pointSize: 4,
          series: {
            0: {color: 'FF0000'},
}});

// Display.
print(NDVIseries);

Can somebody help me to fix the code?