Crystal Reports Group Conditionis not Known

Message

   Hi,

Wondering if anyone can help me.

I am modifying a report that someone else wrote and it basically returns an average elapsed time for a month based on how quickly agents close tickets.

I know have to modify this for a weekly stat but when I modify the formula field to include "weekly" it scomes back saying that the "The group condition is not known"

Here is the formula field pre editing - Round(Average ({@Elapsed Days}, {@dtCompleted}, "monthly"),2)

post edit - Round(Average ({@Elapsed Days}, {@dtCompleted}, "weekly"),2) 

Can anyone tell me why I cant use "weekly" or is my syntax wrong?

Thanks
Sean