An agreement is an understanding, a compromise to get two sides to find common ground. For things to be in agreement they are harmonious or do not contradict each other.
Agreeance is the act of agreement. It is an archaism that is now rarely used correctly.
I am in agreeance with this commentary.
I too am in agreeance with it.
I am in disagreeance.
We were almost finished with “agreeance” and then some damn redneck had to conflate “agreement” with “acceptance” and we’re right back where we started. *facepalm*
Agreeance comes from the Old French agréance and would therefore be expected to be used in areas with (former) French influence. It is also used in New Zealand. But it is commonly viewed as outdated and even wrong in America where “ginormous” is a word.