It's a native command. Type that at the command line. I believe there is also a menu location for it File->Export, maybe. I don't know. I use the keyboard for most everything.
I apologize - it appears Autodesk has removed the command "STEPOUT" from 'vanilla' Autocad at some point. If you're using a recent version, it apparently lacks a once-useful feature for unknown reasons. My last years were spent using Autocad Plant, which had it.
The only workarounds I see is to download a free version of Inventor 365/Fusion/something and open the DWG there and save/export as a STP file.
Agree with JN's last post. The problem with 3D STEP output (and 3D modelling in general) in Autocad is that there was(?) no built-in checks for valid 3D geometry, and you could (for instance) create a part with negative surface area or volume...which causes all kinds of errors in the importing process using 3D modellers.
