If you want to create an OpenStudio Model (*.osm) you need the full OpenStudio Suite.

Don't get confused, there are two different products:

• The OpenStudio Suite (www.openstudio.net / GitHub), from NREL, which has a Sketchup Plugin, the OpenStudio Application, the Parametric Analysis Tool, and the Results Viewer, and the API/SDK. This allows the creation of OpenStudio Model -*.osm file - that you can open in the OpenStudio Application.
• The Legacy OpenStudio Plugin (formerly (?) GitHub NREL/legacy-openstudio) - now maintained by Big Ladder Software if I understand correctly from this answer - which is a Sketchup Plugin to view and modify the geometry of EnergyPlus *.idf files and has some extra capabilities like launching simulations directly from Sketchup and viewing select results.

The OpenStudio Suite is more comprehensive, but the Legacy OpenStudio Plugin is still very relevant if your goal is to work with pure EnergyPlus *.idf files.

One caveat of the OpenStudio Suite is that it cannot reverse translate - meaning translate an *.idf file to an *.osm file - many aspects of an *.idf file. Rule of thumb: anything else than geometry, internal loads and schedules will not be translated. So if you have an already created EnergyPlus model (*.idf), then the Legacy OpenStudio Plugin is a tool of choice!

Additionally, when you use either of these two products, do not use Sketchup's built-in "Save/Save As" command but the one provided by the plugin. For example, if you use the Full OpenStudio Suite, use the "Save OpenStudio Model" button of the OpenStudio toolbar, as highlighted in the OpenStudio SketchUp Plug-in Interface Guide