Thanks for the information. So the error message is not really helpful but the problem is that you are trying to sort on a multivalued field which does not really make sense.

What you should do is add a transient method in PromotionTemplate which will build the sort field (either using a sum, a min, a max... depending on your business use case). Based on what I see of your business case, I suppose a sum would be in order.

Something along the lines of:

@Field
@Transient
public Long getViewsSort() {
     Long result = null;
     for (element in organizationStatistics) {
           your business logic
     }
     return your long;
}

Can you try this and see how it goes?