Exception throws when saving file in Android device in ExternalStorageDirectory

I faced a problem when saving file in the Android device in external storage location.

Xamarin.Forms Android project details:
Target Framework : Android 8.1
Minimum Android version: Android 4.4
Target Android version: Android 8.1

When creating file in the location “Android.OS.Environment.ExternalStorageDirectory” results in exception that the file not found. I also enabled Read and Write external storage permissions in AndroidManifest.xml.

If I changed the Target Android version as “Use Compile Using SDK version”, it working fine.

Can you please let me know why this problem occurs when running application in above mentioned target Android version ?