It looks like the kind of message that calls for you to take the action it suggests. You apparently have a huge spreadsheet, perhaps with some blank cells or columns, perhaps with formatting applied despite having no content at all. You need to delete some of them before you can add new columns. Not much more can be said by way of help.
Describe the workbook to the best of your abilities if what I've said doesn't make sense. Who created it? What does it contain?
