Does not grant me permissions for GPS XAMARIN ANDROID

hi people, my problem is that using the Geolocator in xamarin android does not let me get the coordinates of the device.

[Activity(Label = "@string/app_name", Theme = "@style/AppTheme.NoActionBar", 
MainLauncher = true)]
public class MainActivity : Activity
{
    Button btnGetCoordenadas;
    TextView adressText;

    protected override void OnCreate(Bundle bundle)
    {
        base.OnCreate(bundle);
        SetContentView(Resource.Layout.activity_main);

        btnGetCoordenadas = FindViewById<Button> 
        (Resource.Id.get_address_button);
        btnGetCoordenadas.Click += BtnGetCoordenadas_Click;

        adressText = FindViewById<TextView>(Resource.Id.address_text);
    }

    private async void BtnGetCoordenadas_Click(object sender, EventArgs e)
    {
        var locator = CrossGeolocator.Current;
        locator.DesiredAccuracy = 50;

        var position = await locator.GetPositionAsync();

        adressText.Text = 
        position.Latitude.ToString()+"/"+position.Longitude.ToString();
    }
}

That is the main code and this my manifest.

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android" 
android:versionCode="1" android:versionName="1.0" 
package="PruebaLocation.PruebaLocation" android:installLocation="auto">
<uses-sdk android:minSdkVersion="21" android:targetSdkVersion="27" />
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
<uses-permission android:name="android.permission.INTERNET" />
<application android:allowBackup="true" android:icon="@mipmap/ic_launcher" 
android:label="@string/app_name" 
android:roundIcon="@mipmap/ic_launcher_round" android:supportsRtl="true" 
android:theme="@style/AppTheme"></application>
</manifest>