STL format contains mesh data that can only be viewed but not edited. You need to download the Free Mesh Enabler tool from Autodesk Lab (or maybe also some CAD sites) which allows for conversion of the mesh features to Inventor Base features. The resulting Base features can then be used for further operations including drawings and measurements.
There seems to also be a STL plugin available for Autodesk Inventor with which you can import STLs to the program.