Part of the error message states "All Cells in the Raster Int_Building have the NODATA value." What is "Int_Building" and how did you use it in the viewshed analysis? Any chance when you ran the Viewshed the wrong elevation raster was used (i,e. Int_Bulding was used when DEMBuilding should have be used instead?
Also, a suggestion. Capture a screenshot of the tool inputs just before you run it and post it to this thread. This will help folks see the details, which will then help in evaluating what may be going awry.
Chris Donohue, GISP