Issue:
SQL> ALTER INDEX GTUSER.INDX_PCD_CHARGING_ID REBUILD online NOLOGGING parallel 16;
ALTER INDEX GTUSER.INDX_PCD_CHARGING_ID REBUILD online NOLOGGING parallel 16
*
ERROR at line 1:
ORA-08104: this index object 14944 is being online built or rebuilt

Soultion: 

1. Firstly check the object_id if it exists.SQL> select obj# ,name from obj$ where OBJ#=14944;

      OBJ# NAME
---------- ------------------------------
     14944 INDX_PCD_CHARGING_ID

2. Invoke Clean index function (This function performs a manual cleanup of failed or interrupted online index builds or rebuilds)

declare
msd BOOLEAN;
begin
msd := dbms_repair.online_index_clean(14944);
end;
/

3. Rebuild Again 

ALTER INDEX GTUSER.INDX_PCD_CHARGING_ID REBUILD online NOLOGGING parallel 16;