UWP epPlus - choose directory to save - vb.net

Question

I am making an application in vb using UWP and epPlus along with it to save the current database to an excel file. I would create a pop up dialog box that will allow the client to select the directory they want to save the excel file to. There isn't a "Savefiledialog" in UWP... Ideas?
UPDATE
I tried to use FileSavePicker: if I save it in the principal directory (C:\, Desktop, Documents ...) epPlus doesn't work, but if I save it in another directory (D:\, USB...) it workes... How can I save it in the principal directory?
I post my code here:
Private Async Sub ButtonXls_Click(ByVal sender As Object, ByVal e As RoutedEventArgs)
OutputTextBlock.Text = ""
Dim savePicker As FileSavePicker = New FileSavePicker()
savePicker.SuggestedStartLocation = PickerLocationId.Desktop
savePicker.FileTypeChoices.Add("Microsoft Excel", New List(Of String)() From {
".xlsx"
})
savePicker.SuggestedFileName = "Riepilogo_Test"
Dim file As StorageFile = Await savePicker.PickSaveFileAsync()
If file IsNot Nothing Then
CachedFileManager.DeferUpdates(file)
Dim ExcelPkg As ExcelPackage = New ExcelPackage()
Dim wsSheet1 As ExcelWorksheet = ExcelPkg.Workbook.Worksheets.Add("Sheet1")
Using Rng As ExcelRange = wsSheet1.Cells(2, 2, 2, 2)
Rng.Value = "Text file on Excel here"
Rng.Style.Font.Size = 16
Rng.Style.Font.Bold = True
Rng.Style.Font.Italic = True
End Using
wsSheet1.Protection.IsProtected = False
wsSheet1.Protection.AllowSelectLockedCells = False
Dim filePath As FileInfo = New FileInfo(file.Path)
ExcelPkg.SaveAs(filePath)
Dim status As FileUpdateStatus = Await CachedFileManager.CompleteUpdatesAsync(file)
If status = FileUpdateStatus.Complete Then
OutputTextBlock.Text = "File " & file.Name & " was saved."
ElseIf status = FileUpdateStatus.CompleteAndRenamed Then
OutputTextBlock.Text = "File " & file.Name & " was renamed and saved."
Else
OutputTextBlock.Text = "File " & file.Name & " couldn't be saved."
End If
Else
OutputTextBlock.Text = "Operation cancelled."
End If
End Sub