I think this is because there is a .csc file in the root of your assets. You can delete it, but let me maybe save you some time— I think you might not be using the right version of 2018. After I fixed the csc file, I got other weird build errors that went away when I used 2018.2.20 (2018.2.21 also seems ok).
I then ran into a different problem, where shaders won’t load correctly. I’ll make another post about that so as not to hijack this one.