You're getting that error because the STC counter on the 6733 does not support Two-Edge Separation Measurement. The link below has a comparison of the STC and TIO counters:
http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/4501544a6b93bf348625672e00755db9?OpenDocument
Hope this helps.
Regards,
Erin