The Insert File option only shows SKP, PNG, and JPG, STL seems not to be an option. You can import STL into SketchUp, but unfortunately only with the 2018 version of Pro, there isn’t a 2018 version of Make.
There are online 3D file converters, such as this one: http://www.greentoken.de/onlineconv/
With that you could convert STL to Collada, and bring that into Make 2017, to make a SKP that the online version can use.
Where did the STL come from? Often there are alternate formats for a model, you could take a Collada version and skip the online converter tool stage.