Agreeance is a word that seems to be continually reinvented and recycled. The term first appeared in the 16th century and enjoyed its greatest popularity in the 18th and early to mid-19th centuries. Since then it has fallen into disuse. Most likely unaware of the word's history, modern writers who recoin the term seem to like how it sounds, even though it adds nothing in meaning to its workhorse counterpart, agreement. The suffix ‐ance indeed is used in English to form many nouns from verbs: for example, appearance, acceptance, performance, and remittance. So while there is no rule preventing the formation agree + -ance, the coinage may sound quaint or pretentious to some people.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019