This is an issue when trying to use the Legacy OpenStudio plug-in with SketchUp 2016. According to @macumber on the Github issue posted HERE, this is something that changed within SketchUp and not the plug-in.

What I have done is use the same Legacy OpenStudio plug-in, but for a previous version of SketchUp (SketchUp 8, SketchUp 2014, etc.). That has worked fine without any issues.

I hope this helps,

Aaron