The code have an error and do not run

Try this.

Private Sub Workbook_Open()

Dim mydir As String, Pfad As String, Tag As String

With ActiveDocument
    With Application.FileDialog(msoFileDialogFolderPicker)
        .AllowMultiSelect = False
        .InitialFileName = "G:\Shareplan\test_31686\Plan\07\"
        If .Show <> -1 Then MsgBox "No folder selected! Exiting sub...": Exit Sub
        mydir = .SelectedItems(1)
    End With
    Pfad = mydir & "\"
    Tag = Format("yymmdd")
    Application.DisplayAlerts = False
    .SaveAs Pfad & "contrib_per_payroll_check_" & Tag
    Application.DisplayAlerts = True

    'Application.GetSaveAsFilename
        
    .Close
End With

End Sub