Reported: "a hundred euro" should be acceptible. Replace "a" by "1" has no advantage at all. Insisting on euros instead of euro may be correct English, but many of the learners are non-English speakers where it is important to transfer the meaning of Finnish words and sentences and also test for the understanding, not for perfect English.
Insisting on "euros" instead of "euro" is a mistake, not just a bother! https://english.stackexchange.com/q/13551/73094 - both euros and euro are grammatically correct in English.
Now it accepts a hundred euro, but it says "You have a typo" and suggests a hundred euros instead. This is annoying.