Flashback

I am currently trying to use the execute DBMS_FLASHBACK.ENABLE_AT_SYSTEM_CHANGE_NUMBER in a sql step in an application engine so that in sql steps executing below it hopefully should get the data in the tables for what it was at the instance for the SCN. I've tried using the execute DBMS_FLASHBACK.ENABLE_AT_SYSTEM_CHANGE_NUMBER (number goes here) and was wondering if It needed the "execute" in front of it or not. I first tested this out in SQL DEVELOPER and get the following error:

BEGIN DBMS_FLASHBACK.ENABLE_AT_SYSTEM_CHANGE_NUMBER(66769821707); END;