You are not defining the FIle name correctly. ActiveDocument.Name
Code:
Sub rtf_test()
Dim strFname As String
Dim oDoc As Document
Dim strPath As String
strFname = "Filename"
Set oDoc = ActiveDocument
strPath = ActiveDocument.Path & "\"
oDoc.SaveAs2 FileName:=strPath & ActiveDocument.Name & ".rtf", FileFormat:=wdFormatRTF
End Sub