"For a market to exist there must be scarcity" (T/F) The answer provided was 'true', that scarcity was necessary in the existence of a market. But isn't that not always true? For example, there's a market for canned air, and air is most definitely not scarce.

Is the statement true or false? And if true, what about in the case of canned air?
I agree that air is not, in any meaningful sense, scarce. But there is no market in air! There is a market in canned air, but that's different because canned air is most definitely scarce.

The can is made of metal, which is not in infinite supply. If you use metal to produce a can of air then you must take that metal away from production of, say, airplanes or paperclips. Alternatively, you could just mine more metal. But then you would need to hire more miners, so you are taking workers away from some other industry.

Moreover, the can is manufactured by scarce labour. To manufacture the can, you also need machines and tools that are in limited supply abd could be used to make soup cans instead. The machines stand in a building on land that is in scarce supply and could be used for housing or farming. To get the metal to that building, you need scarce trucks, drivers, and fuel.