Replacement cooktop wiring question
We haven't heard back from OP, but I suspect what's going on here is that the range uses metallic conduit all the way back to the panel, and the conduit is the ground, accessible via the green #10-32 screw in the metal junction box. That's a legal setup; OP is good to go.  

This is a modern range which does not require a neutral wire.  Historically neutral was used for nothing but the oven light and the clock, so it's not hard to get rid of.  If there's a neutral in the supply cable, it should be capped off.

In the off chance this is an obsolete 3-wire circuit with a cable, if the neutral wire is a bare wire, it can be permanently converted to a ground.  This is irreversible.  Inside the panel it would need to be moved from the neutral bar to the ground bar (on a main panel they are probably the same bar). A white neutral wire can never be converted to a ground.