Foreign Key Constraint Issues
Adding a foreign key constraint ona table locks it since it needs to verify the data with the primary data source for the constraint to be enabled automatically.
One way would be shutdown the instance (release locks on all db objects). Open the DB in restricted session, setup the constraint and open to public.
Option 2: If you have any monitoring tool kill the offending session (minimal impact), wait for about 3 minutes for SMON to clean up the killed session and rerun your statement.
Hope this helps.