Hello!,
Take a look to *.F06 file located in the same directory of thye NX *.prt file as well as *.sim and *.fem files. Open the *.F06 using any text editor like Notepad.exe or wordpad.exe and search the word FATAL, there you will see the description of the nx nastran error.
Best regards,
Blas.
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Blas Molero Hidalgo
Ingeniero Industrial
Director