Getting "java.lang.RuntimeException: Distinct columns cannot exist in Aggregate "

select platform, 
        collect_set(user_auth) as paid_types,
        count(distinct sessionid) as sessions
    from non_hss.session
    where
        event = 'stop' and platform != 'testplatform' and
        not (month = MONTH(current_date()) AND year = YEAR(current_date()) and day = day(current_date())) and
        (
            (month >= MONTH(add_months(CURRENT_DATE(), -5)) AND year = YEAR(add_months(CURRENT_DATE(), -5)))
            OR
            (month <= MONTH(add_months(CURRENT_DATE(), -5)) AND year > YEAR(add_months(CURRENT_DATE(), -5)))
        )
    group by platform

java.lang.RuntimeException: Distinct columns cannot exist in Aggregate operator containing aggregate functions which don't support partial aggregation.