difficulty vs difficultness

difficulty and difficultness both are nouns.

Word	Noun	Adjective	Verb	Adverb
difficulty	Yes	No	No	No
difficultness	Yes	No	No	No

As nouns, difficulty and difficultness are synonyms defined as:

difficulty and difficultness: the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result

difficulty (noun)	difficultness (noun)
an effort that is inconvenient	the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result
the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result