Error in creating trigger
I assume new.follower_id is supposed to be:

select follower_id from folow where circle_id=new.circle_id;


I don't think this will work. If you want to use this approach you need to declare a variable for follower_id and select into that. However, a better approach (IMO) is to do:

INSERT INTO activity( uid, performed_activity_id, activity_type_id ) 
    VALUES (new.uid, new.event_id,  '1');

insert into notification_table (sender_id,object_id,receiver_id)
    select new.uid ,new.event_id, follower_id 
    from folow where circle_id=new.circle_id;