I have checked your workbook and not found any <0 values.
If [Sales]<0 then 'No Sales'
elseif ISNULL(str([Sales])) then 'No Data'
ELSEIF [Sales]>1 and [Sales]<=500 then 'Medium'
Refer above formula will give you null as no data and <0 as no sales.
Empty Row.twbx 370.5 KB