Translation:Peace is the biggest achievement of human.
With the word selection, I said "the biggest human achievement" and was wrong =(
"Peace is the biggest achievement of human", is the most direct translation, but the word 'human' in English is a countable noun and requires an article if it is singular, or must be in plural form if it is not. In this case it should be in plural form. The English grammar in the translation is incorrect. It should be, "Peace is the biggest achievement of humans (or people)"
Human can be an adjective or noun. If adjective, singular. In this case it appears to be a noun so requires plural.