Humans = plural

Software is not perfect, and in this case, you are correct. Humans is a plural noun, with the singular form of human.

Human is a particular difficult word to handle using a simple set of software rules. Its primary sense is that of an adjective -- human being or human beings, human nature. In its secondary sense, it is a noun. While the noun human is a singular (with humans as its plural), its use as an adjective often links it with plural nouns. While we humans can understand the context and understand how human is used in a sentence, software cannot. As a result, the software may suggest a rule that is not applicable to the usage.

I hope this helps.

PS, you should complain directly to Grammarly Support. The support staff does not read this forum.

You are right. Human as a noun is countable so "Humans" is plural form of noun "Human".

Ref: http://www.goodwordguide.com/dictionary/human#noun