Generic tables must contain at least three fields... does your generic table contain at least three fields? If you so, you may want to include your code and sample data, otherwise it's quite difficult to guess what the problem might be.
The Crosstable prefix creates a generic table from a crosstable. You need at least three fields in the created generic table. This is actually covered in the documentation, so you may want to have a look at that since I don' think any explanation I'd provide would be better than the one they give: