I had this same problem this week.
I don't think blender 2.8 will knife project if you are using a 3D object as the cutter.
In blender 2.8 when you select both objects @theluthier they both go into edit mode (unlike in previous versions of blender) and I also got the message it would not work and there was no cutting plane selected.
Is there a new workflow in blender 2.8 with this tool. I could not really figure it out.
I ended up just doing a boolean intersection lol because I couldn't get it to work.
I also couldn't figure out how Knife Project works in 2.8, but a few days ago I stumbled upon this: https://www.creativeshrimp.com/detailed-cutouts-blender-tutorial.html
Man I guess it does work still. Did he just deselect everything prior to project knifing spikeyxxx ? Does this mean that this will work for 3D shapes knife projecting as well? I guess I will have to play.
blanchsb I think he said it doesn't work with Faces, only with outlines; it also doesn't work with 3-dimensional shapes, at least the result is unpredictable, but it doesn't matter what is selected in Edit Mode:
To be honest, I have used Knife Project only once, so far and that was in this course;)
I encountered this problem attempting to slice a mesh with a plane at a 45 degree angle. I gave up after about 10 minutes of trying to figure it out, however, I was able to achieve the same result by using the bisect tool.
