Translation:I do not agree with you on this.
That's really splitting hairs. I am yet to encounter a context/situation where "I do not agree"≠"I disagree". What you are talking about here corresponds to English "I don't fully/entirely/completely agree". Meantime mere "I don't agree" is always understood as a statement of disagreement.
There is no sensible argument for not accepting it here.
Duolingo likes to be exact. "I don't agree" means either I disagree or am neutral about it (not against or for your point). "I disagree" means only I disagree. When people get it wrong they can come to this thread and learn something... however useless it may be.
I could see the example "I don't agree with the way he handled the situation but it was effective." Here the speaker is not condemning or disagreeing with the actions, merely not supporting them.
I entered "I do not agree with you on this," then looked for "point" in the word bank but didn't find it. So I entered "marking" assuming Duolingo was being obtuse, as it sometimes is, and was marked wrong. The idea that it could have expected me not to translate "Punkt" at all didn't cross my mind. Pretty irritating.