A higher level than you have lol. Mixing bleach and ammonia creates Chloramine and chlorine gas which are toxic when inhaled but not mustard gas which causes chemical burns on contact and is generally far more dangerous.

This reaction is fairly easy to trigger on a basic level but producing enough quantity to use in a terrorist attack is not trivial. You will also need the skills and equipment needed to do so safely, as well as methods to store and deliver the gas to your victims. All in all this requires way more chemistry and engineering than your average person will be capable of without extensive study and planning. Yes, the necessary information and materials are likely available, but this is far from easy.

Compare with a vehicle-based attack. Vehicles are widely accessible to almost anyone—most people already own one and if not almost anyone can rent one. All you do is point the vehicle at the targets and press the accelerator. In fact it’s so easy that there’s tons of vehicle-based violence that doesn’t even make the news because it is just “normal” for people to be killed this way.

On the other hand I can’t think of a single chemical attack that was committed by non-state actors. I wouldn’t be surprised if it has happened at some point but it is very rare.