Dropouts seem to be tough for beginners. Makes me wonder about all the other joints though. That nokorode flux looks like an electronic flux. It's unlikely to be the right temperature range.
You don't have your location set on your profile. Most local welding stores don't have brass flux. It's possible they may have a powdered borax type, which is a pita to use. You can get Gasflux type b from most of the framebuilding suppliers.