Porosity and fiber diameter with imagej

Hello, I’m analysing an image of nanofibers in imagej. I want to measure the porosity and diameter of the fibres but I can’t do it because I get errors.
First I have measured the scale of the image with set scale, and then when I go to plugins->diameterj->diameter segment I get this error that you can see in the photo.

What should I do? Someone can helps me?

Thank you!

Malen