Translation:I don't take taxis.
To understand には it helps to first consider what function に and は have on their own.
は sets the topic or scope of discussion. Xは... is sort of like "As for X..." or "When it comes to X..." - you should imagine a sort of more general context and we're zooming in on X. It can sometimes have a mildly contrastive effect, putting emphasis on the fact that X is what we're discussing and not something else.
に sets the target location of an action (either in space or time). It's like "at" or "in" or "on". When you throw a ball to そこ, it's そこにボールを投げる, whereas if you're standing at そこ when you throw, it'd be そこでボールを投げる.
If we combine these, we're placing the focus of discussion on the target location where things are happening.
Here, we're putting emphasis on "taxi" as being what we're talking about, at the same time as making it the target of 乗りません that is, the thing we're not taking. Probably in English we'd express this emphasis just with a bit of intonation - some extra stress on the word "taxis". A sentence like "As for taxis, I don't take them." gives the right idea too though.
Basically right and good explanation. There are two things to add: - は can be combined with any article. But in case of が and を, the original particle will be replaced by は, any other particle will still be used before the は - As you mentioned in the first explanation, は sets the topic or scope. But at the end you write emphasis. That's not what I would say. The topic is the thing/person/animal/plant you are talking about in general, not the new or important information that you provide. Therefore, the use of は can often have the opposite effect of an emphasis (see all the explanations of the difference between using は and が when the topic is in the role of the subject). Furthermore, the topic is so unimportant that after it is made clear once, you can just leave it out in the following sentences, similiar to how we use pronouns instead of using the same word again in western languages. That's why we have all there sentences where we do not know the subject or object - because the topic is omitted because in a real conversation it would be (hopefully) clear from the context or because it was previously mentioned. Therefore, the focus of the conversation is often the part that is not the topic. A possible reply could be 私が乗ります。 - I take "them"/the taxis. (With a stress on I - and the topic "them'" is still the one marked in the previous sentence, since there is no new topic provided. The stress would be on "I" here.) This is just my understanding, correct me if I misunderstood something.
Gonna copy-paste my answer for a similar question here.
the other comments are not wrong but here's a more straightforward answer: typically you add は after に if the verb (in this case "ride") is in its negative form. (e.g. "I do not ride taxis" - "タクシーにはのりません") You usually don't add は after に otherwise, unless you specifically want to mark "takushii ni" as the topic, although I can't think of a case where it would make sense to do so.
Duolingo doesn't really do a good job of explaining verb negation or just particles in general. Here's a pretty good explanation of how you can use particles in negative form: https://www.learn-japanese-adventure.com/japanese-particles-change.html
Wait, "take" is appropriate here? I mean, I guess I understand it, but I thought it was translated into "ride." "Take" implies a specific destination, while "ride" doesn't. Like, "I'll take a car (to go to the hospital)" as opposed to "I'll ride a car" which isn't shortened English and just means what it means. The purpose isn't implied in the latter sentence. Maybe the person needs to go somewhere, but maybe he just wants to go for a ride. Admittedly, though, "take" can be short for "take out for intended use," too. I think this may come from wondering which route to "take," so it implies making a choice on how to go about achieving something specific (such as arriving at a particular destination.) So it answers "how will you do it?" rather than just "will you do it?"
This may seem like English semantics, but that's because I'm more interested in understanding the thoughts behind the words than 1:1 translations (not that the latter isn't also important. I just think that if you understand the reason behind the sentence construction, and the idea behind what a person's saying, then it sets you up for learning the rote stuff more easily and you have a deeper understanding altogether. So, I don't what to just know "what" but "how" and (most importantly) "why." After all, all sentences start as ideas/thoughts that want to be expressed. The sentences that are formed from them are just a way of achieving that expression.)
Can somebody tell me why my sentence was not right? I've checked my kanji from Takoboto and it seems to be correct (to get on). Am I not supposed to use kanji that are not taught yet? I do it on my PC with my keyboard (not using the word bank) and I feel like I should be able to use either Hiragana or Kanji..