Your document also saves as strFilename, which doesn't exist in your code. As Paul indicates the document should be docx format and the code is in the template. The code when edited to
Code:
Option Explicit
Private Sub Document_New()
Dim intChoice As Integer
Dim strPathName As String
intChoice = Application.FileDialog(msoFileDialogSaveAs).Show
If intChoice <> 0 Then
strPathName = Application.FileDialog(msoFileDialogSaveAs).SelectedItems(1)
ActiveDocument.SaveAs2 Filename:=strPathName, FileFormat:=wdFormatXMLDocument
End If
End Sub
saves as docx and the document can certainly be opened without error.