I bet you'd need a vehicle the size of a locomotive to (1) carry the processor that converts the ammonia tablet into hydrogen, and (2) processes the hydrogen to provide energy to drive the vehicle.
Even if you can catalyze ammonia to yield hydrogen and nitrogen, this still smells like snake-oil.
Well, no. The processor that converts the ammonia to hydrogen is probably nothing more than series of metal screens, made out of some kind of catalyst. Or it might resemble your catalytic converter. Other than a pressure regulator, nothing that is not already in your engine is required to burn the hydrogen, in fact you can do away with the fuel injectors, fuel pump, fuel pressure regulator and fuel filter. It would be no different than a conversion to burn natural gas, which many fleet vehicles already do, although the ignition timing curve would probably have to be reprogrammed a bit.
Currently a common process for creating ammonia, the Haber Process, reacts Nitrogen (from the air!) with hydrogen () in the presence of a porous iron catalyst to form ammonia. While hydrogen is most commonly made by steam reprocessng natural gas, that is not the only way to do it. Electrolysis of water is another method that can be used on an industrial scale, you just need enough electricity, as from a nuclear generation plant. Still another method uses just heat and chemical reagents, which get recycled in the process. The input is water and heat, the output is hydrogen and oxygen.
Both hydrogen and ammonia are not nice substances to handle. The former is hard to contain, because the molecule is so small, the latter was once widely used in refrigeration systems as the coolant, leaks and accidents resulted in many deaths. However gasoline is also not a nice substance to have around, but we manage that by the thousands of gallons, and that's *after* it has left the pipeline. Refineries routinely suffer explosions and fires.