There are a few reasons why this might be happening.  Once Setup performs some checks to make sure that only the right data is pasted into the tool.

- Does your data contain any letters or symbols in the price fields?  These fields must be numeric only, e.g. remove any £ signs

- Check the headings of your spreadsheet.  Once Setup needs the first row to be the width (in mm) and the first column to be the drop (in mm). Remove any letters such as 'mm'