A common cause of this error is that the ".prt" isn't from NX. Pro/E/ wildfire/Creo use( or used to have?) ".prt" as file name extension as well.
I have seen a couple of cases where people have received files with this extension and by that concluded that it should be NX. I have opened a few such files in notepad, and among the "garble text" it is often possible to see "parametric technology".
Similarly, if you have 7-Zip installed and right click on the prt file and choose "7-Zip > Open Archive". If it's an NX part file you'll see a folder titled "UG_PART", amongst other things that you might recognize, like qafmetadata.