I tested and the script run ok!
INSERT INTO HISTORICAL_CAR_STATS (HISTORICAL_CAR_STATS_ID, YEAR,MONTH,MAKE,MODEL,REGION,AVG_MSRP,COUNT)
WITH DATA AS
(
SELECT '2010' YEAR,'12' MONTH ,'ALL' MAKE,'ALL' MODEL,REGION,sum(AVG_MSRP*COUNT)/sum(COUNT) AVG_MSRP,sum(Count) COUNT
FROM HISTORICAL_CAR_STATS
WHERE YEAR = '2010' AND MONTH = '12'
AND MAKE != 'ALL' GROUP BY REGION
)
SELECT MY_SEQ.NEXTVAL, YEAR,MONTH,MAKE,MODEL,REGION,AVG_MSRP,COUNT
FROM DATA;
you can read this article to understand more! http://www.orafaq.com/wiki/ORA-02287