I have the exact same problem where the SDF mode can't be checked! Unity 2017.4.15f1, v1.8.10, I can't check SDF mode, and I can't check "Pixel Snap" either (although I don't know what that does).
It just immediately toggles itself off.
I'm just using the built in prefab, right click create -> 3D Objects -> Super 3D Text, and no dice. I've got an SDF atlas generated, but it also doesn't work on a regular font (Arial) either.