Dear Kien,
I was not able to get to your files. Did you remove them? In the second file, did you add any supports? If you do not have any support in the model the program can not compute the longitudinal stress.
Another reason might be related to the Eigen solver used to find the natural frequency of the vessel. You can try to turn it off from Help menu -> Uncheck Eigen solver. Then try again.
If you still have problems then re-upload your files or send them to support at coade.com and put it to my attention.
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Best Regards,
Mandeep Singh
CADWorx & Analysis Solutions
Hexagon PPM