I am getting the Union Error #000229 - Unable to open source layer
I have tried several solutions to no avail:
1. Copying the datasets to another file location (desktop, shared network drive, etc.)
2. Renaming the geodatabase and the source files
3. Exporting the source files as shapefiles (.shp)
4. Completely restarting a new project and inserting the files and retrying the Union.
NOTHING has worked! Its ironic because this is simple geoprocessing task that was set up in a tutorial through ESRI.
I have contacted ESRI support and waiting for a call. I will repost if I get a solution.