create index on Materialized views failed with 'resource busy...' error

Hello All:
I try to create index to speed-up one report at warehouse DB. 

However, I encounter ora-00054 error when creating index on it.

SQL>connect prod/
SQL>connected

SQL> create index SR2_ACTION_DATE_ID on SYSTEM_REQ_AUD2_MV('ACTION_DATE') tablespace QINDEX;
create index SR2_ACTION_DATE_ID on SYSTEM_REQ_AUD2_MV('ACTION_DATE') tablespace QINDEX
                                                      *
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified


SQL> desc system_req_aud2_mv
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 ACTION_DATE                               NOT NULL DATE
 INIT_REQ_STATE                                     NUMBER
 OID                                       NOT NULL NUMBER
 OID_SYSTEM_REQ                            NOT NULL NUMBER