DATEDIFF has 3 params to pass. 'Interval', 'Date1', 'Date2'. Looks like you have Date1 and Date2, but not how you want the "Difference' returned in.
You probably want day. so try 'DATEDIFF(day, CURDATE(), duedate) AS dayslate'
Options:
year, yyyy, yy = Year
quarter, qq, q = Quarter
month...