Classic,
I am not that familar with Sketchup ad may be wrong on this, but don't think that the free version will save the file as a ( .STL ).
There are addditional converters out there for Sketchup, sorry but don't have a link to them.
You may want to post a converted file when you convert it so someone here can have a look and see what it does.
If memory is correct there was a problem with the some them ....Just a heads up.
Maybe someone else will reply who is more knowledgable on it's use.
RICH