Error in expression - Set Analysis

[#1]

Hello

I am getting the error  for the following Expression

Can you guys please help me to understand the reasons?

I Need the no of Customers (KP.EMM-ID)

where the Booking Type = 'reward order' and the max no of orders <=3

Many thanks

Sonja

[#2]

Yes, many thanks, Sebastien Fatoux!!