What is the noun form of "erratic"?
ODO shows erraticism as the noun.
adjective
not even or regular in pattern or movement; unpredictable:erraticism
noun
It is related to error in that they both stem from Latin errare, to stray (from the truth in one case, from the straight and narrow in the other).
Increasing Monsoon unpredictability and erraticism
Though the erraticism of India’s monsoons is fairly documented, research provides ample proof that global warming has added to the unpredictability and erraticism of the same.
Erratic is also a noun.
erratic (noun)
an erratic or eccentric person.
Geology . an erratic boulder or the like.
OED returns several example sentences with erratic and erratics as noun forms.
Eg. Bacchic priests.... erratics or wanderers
Erratic is derived from a Latin participle of erro, the verb for "to wander, to stray" from which error is also derived. One might gloss it as "tending to stray from a regular course." Even error has this original sense of being "off-course."
As a comparison, the word "eccentric" literally means "off-center," and so metaphorically "wandering from a circular or regular course." There are two obvious noun forms. An "eccentric" is someone or something that is eccentric. Eccentricity is the tendency to be eccentric.
Nouns derived from erratic, however, tend to be non-standard: you are unlikely to hear anyone speak of "erraticness," "an erratic" (as in a person, though there are specialized vocabularies where this form might be used), or "erraticism." One is far more likely to use "irregularity" or "instability" or some other word.