Inconsistency on DL. “The people” was accepted as a translation of “a população” in a similar sentence but rejected here.
And given all the intelligent comment on here on eat/eats, it’s a shame that DL is still rejecting “eat”. To repeat the point, “population” used as in this context is a collective noun in English and consequently can be followed by either “eat” or “eats”, with UK English tending to prefer the former and US English the latter.
Your English was correct don't worry. In British/Irish English, "The population eat rice is correct. You will notice the same thing when people talk about football teams. "France are an amazing football team" or "Arsenal have problems defending" are completely correct and it would sound really strange to use "is" or "has" here. In America the opposite is true. Duolingo is being very American-centric here by not accepting the third person plural answer here TLDR: "Population" can be both a plural or singular noun https://www.englishforums.com/English/PopulationSingularPlural/bxpxd/post.htm