I am not sure where you are getting the information that it was considered a failure. While not perfect it was relatively well received at the time and still holds interesting nuggets of information. I would agree that it is not nearly as well known or considered as the more famous document of the same title; however, it is not considered a failure so far as I know.

The text of Machiavelli's Art of War is freely available online in several places: https://www.marxists.org/reference/archive/machiavelli/works/art-war/

It takes the form of a dialogue, a classical literary form where two or more characters are written as if discussing the subject, such as in Plato's Republic. Machiavelli's text has less to do with discussing the ideas and tactics of war, and is mostly interested in discussing how to form, train and maintain an army.

If you have read much of Machiavelli's work, you'll notice that when he talks about military matters one of his favourite topics is the ineffectiveness of mercenaries. He, like several other Humanist scholars, believes that an army composed of mercenaries is a recipe for disaster. He advocates instead something similar to the Roman model, of a citizen army that is professionally trained, equipped and drilled. A concept that he also espouses in the Art of War.

In this sense, you'd be challenged to argue that Machiavelli's ideas in the Art of War are a failure. in the next couple of centuries various nations began to form, train and equip professional armies composed of citizens, eschewing mercenaries, and organising themselves along lines not dissimilar to the Legions of old.

That said, I doubt that Machiavelli's work was the foundations of this sea-change, just merely something that, like much of his work, was remarkably long-sighted, and which proved to be a useful basis for innovators who were widely read.