This is a better topic for the analysis group rather than system admin, but with that being said ... What version of Creo are you using?
I believe the export to Nastran deck was only available in Simulate independent mode and PTC retired that with Creo 3.0. Creo 2.0 was the last product to support it.
There is a reason they sunset that product it was not user friendly. The one time I exported an analysis to a Nastran deck it has way too many elements and the Nastran folks scoffed at me. Its possible I just didn't know how to set up the export properly, but like I said, it was not a user friendly product.
