What happened to Oracle's COUNTDISTINCT() function?

I need the count of unique values, and am trying to use Oracle's [COUNTDISTINCT()](https://docs.oracle.com/cd/E93962_01/bigData.Doc/eql_onPrem/src/reql_aggregation_countdistinct.html) function to obtain that:

select COUNTDISTINCT(a.m_label)
        from user_rep a, user_group_rep b, trn_grp_rep c
        where b.m_user_id = a.m_reference
...

This results in ORA-00904: "COUNTDISTINCT": invalid identifier... Using plain COUNT() works, but is not returning correct results. What am I doing wrong? What'd be an efficient work-around?