Error adding a virtual column to a table

SQL> desc event_audit_t_bak;
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 OBJ_ID0                                            NUMBER(38)
 REC_ID                                             NUMBER(38)
 OBJECT_DB                                          NUMBER(38)
 OBJECT_ID0                                         NUMBER(38)
 OBJECT_TYPE_NAME                                   VARCHAR2(255)
 OBJECT_REV                                         NUMBER(38)
 OBJECT_TYPE_ID                                     NUMBER(38)
 OBJECT_TYPE                                        VARCHAR2(255)

SQL> ALTER TABLE event_audit_t_bak ADD (account_obj_DB int , account_obj_ID0 int , account_obj_TYPE_ID int, account_obj_TYPE generated always as (SUBSTRB(PIN_VIRTUAL_COLUMNS.GET_OBJECT_NAME(account_obj_TYPE_ID),0,255)) virtual , account_obj_REV int );
ALTER TABLE event_audit_t_bak ADD (account_obj_DB int , account_obj_ID0 int , account_obj_TYPE_ID int, account_obj_TYPE generated always as (SUBSTRB(PIN_VIRTUAL_COLUMNS.GET_OBJECT_NAME(account_obj_TYPE_ID),0,255)) virtual , account_obj_REV int )