I find it quite awkward too, but not wrong. For me, that is mostly because of the S on toward. In American English toward is much more common that towards.
Other than that, it is just not a word choice we would use. In this instance, I'd say, "Someone is walking this way." I think of toward/towards as a word used to point away from oneself. "Toward the door," "toward the sun," "toward the the crowd," etc.
I'm also an American English speaker, but I prefer towards.
Either way, I think the problem is more the "towards here" than any other part. We don't usually use them together. If you're talking to someone else in the same place with you you'd probably say "towards us", right? If they're on the phone or something I'd usually say something is walking "towards me".
I wouldn't use "towards there" either, I'd prefer to use a pronoun like them, him, her, or it. So I think maybe it's just awkward English to use towards with an adverb like here or there, it should be used with nouns or pronouns.
I think the translation problem comes from the weirdness of the words "here" and "there" in English. Even though we can think of a place as "there" we usually don't use it as a pronoun for the place. That's why you don't replace "I am going to the grocery store," with "I am going to there." Instead you just say "I am going there," in which "there" is describing "going" and is not its direct object. I'm not sure though, someone who actually studies English grammar is welcome to school me.
"Here" as well as "over here" can be construed as a place or a direction, so the English version would stay ambiguous. On the other hand, you could and probably would say "coming" and leave the manner of motion underspecified. In a natural translations, very often some information has to go. Wouldn't it be better to accept all these variations and just offer an alternative translation with the OK at the end?
Prepositions are a pain because they vary from place to place. I speak Australian English. I would never use the example sentence. Because clearly I am here, I could say "someone is walking towards me". I could also say "towards this place". The meaning is the same but the emphasis is different.
"Someone is walking over here" does not imply directional movement for me, so it does not work as an exact translation.
It is ironic how a language site like this promotes mirror translations, as well as mistranslations. This is not the only example. But let us remember that duolingo offers its content free of charge. And the duolingo method works just fine! So do not get your knickers in a twist. By the way, what is the mirror translations in Turkish for this one? :-)
I translated it as 'Someone's walking here'. I am an natural English speaker, so the 'towards' seems odd and superfluous. As the speaker in this example is at a place, then if someone is walking in this direction/towards the speaker, they are coming 'here'. The clear implication is that they are walking towards the speaker, but 'towards' does not need to be said. On the other hand, if the someone was walking away from the speaker, they would be walking 'towards' there or somewhere else