You get a Revit External Tool Failure error when trying to import or export an ISM repository from Revit
Steps to Resolve
If you are having a problem with ISM on your machine I would recommend doing a clean install of both the ISM Revit Plugin and Structural Synchronizer.
If the error persists it is recommended that you do a clean install of Revit. Here is a link to the clean install instructions on the Autodesk website.
Clean Installation of Revit Structure
ISM Revit Plugin