Unable to Group a Number Range in Pivot Table (Excel 2016)

Hello! I've been working on this all day and I've checked out many internet search questions regarding this issue, but I can't find a way to solve my problem - hopefully someone here has the answer!

I have attached my workbook, with the two tabs (one with the raw data, and one pivot sheet). In my first column, you will see that there are a bunch of numbers (they represent numbers of days past due). I have made sure that each of those numbers are formatted as numbers with 0 decimal points. There are no blank cells in my column. Whenever I try and group, I get an error message box saying this selection is unable to be grouped. Any suggestions?

Thank you so much!