Here are three possibilities
1) There are no waveguides at the end of the wafer, therefore no output waveguides
2) There are waveguides at the end of the wafer, but those waveguides have no modes.
3) Additional Output Data dialog box can request PIW in one of three BPM simulators: 2D, 3D, and 3D Anisotropic. The PIW was requested on one simulator, but the project was run with a different simulator.