You can't say eins Ding for "one thing". You have to say ein Ding whether it would be "one thing" or "a thing" in English.
There is a special form of the nominative/accusative neuter (eins) for counting, and that is a slight move towards the way English does it, but it's not quite the same. If you want to say "not one", meaning not a single person, you have to say nicht einer. Gender and case determine the form (which will be ein, eine, eins, einen, einem, einer, eines) much more than any distinction between article and numeral, which is almost entirely absent.
There is an accent on "Ένα" but it isn't very visible. What kind of exercise was this so we can check to be sure there isn't some kind of internal error? Or perhaps find a way to make the accent more prominent.
Missing or incorrect accents are identified by the system but sentences are not rejected. (expect in the few cases where the meaning of the word is changed, but that's for later in the course.)