Inconclusiveness is the fact of not leading to a clear decision or result — the quality of being inconclusive. You might talk about the inconclusiveness of the evidence, of a report, or of early data. It is a noun built from the adjective, and it is quite formal, more a word for writing than for speech. Though actually, English speakers often prefer a simpler shape: “the results were inconclusive” is much more common, and usually clearer, than “the inconclusiveness of the results.” The long noun is there when you need to name that quality as a thing.