Why I can't write in Assets folder from XAML Project?

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XAML

I read a xml file from assets. Now I want to replace modify xml file, but when I want to wirte in xml file throwing exception:

An exception of type 'System.UnauthorizedAccessException' occurred in mscorlib.dll but was not handled in user code

Additional information: Access is denied. (Exception from HRESULT: 0x80070005 (E_ACCESSDENIED))

I can write xml file in C:\Users\Sergiu\Documents, but I want to write xml file in Assets folder from project.

 private async void CreateFILE(string file_name)
        {
             
           folder = await Windows.ApplicationModel.Package.Current.InstalledLocation.GetFolderAsync("Assets");
           
            sampleFile = await folder.CreateFileAsync(file_name, CreationCollisionOption.OpenIfExists);

        }
        private async void WriteFILE()
        {
           await Windows.Storage.FileIO.WriteTextAsync(sampleFile, xml_file); 
//here throw exception          
// xml_file is xml file contain
string xml_file = @"<?xml version=""1.0"" encoding=""utf-8"" ?>\n<Players>\n";
//...
xml_file+="</Players>";
        }