There are a couple of ways to generate random sample data. Which one are you using? What parameters are you setting in the dialog boxes and what has been modified in the header? If you can provide a step by step workflow that could help.

I'm using "Generate Random Sample Using Ground Truth Data" in ENVI 5.0 (32bit). I select the Classification raster, receive the progress bar for computing class distribution, and the receive the error. There are no additional parameters/boxes. The header was modified to change some of the class names (text). This is the only option I see for creating a random sample, and search only turns up this result in the toolbox. If there are more I'd love to try them. thanks again