"Solid, liquid, and gas."
Translation:Στερεό,υγρό και αέριο.
We're taught not to use it before και (points are deducted if you do). In fact it's a whole category of conjunctions that cannot be paired with commas. While not explicitly mentioned in grammar books, all the examples like the sentence above do not have commas. Also teachers always make sure to drill this into our heads. :)
I noticed that I lost a mark for the extra comma
That would surprise me; Duolingo completely ignores commas, as far as I know.
Do you happen to have a screenshot showing your answer being marked wrong for a misplaced comma?
I'm pretty sure that if you got marked wrong, it will be for something else than a missing comma. But people nearly never show screenshots so it's impossible to say what it was.