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.

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

They typically have some kind of grammar lesson for each skill on the web version. Kind of like a short write-up of important things to note before you take the lesson. They have yet to do something like that for iOS and Android, though.

You can use lingodeer its very good in grammatical part

Yes to the first question. No to the second one. タクシーには乗りません means: As far as taxis, i do not take one. The wa really just shifts focus to taxi and away from the verb phrase 'takushi- ni norimasu'

I wonder this as well. Duo accepted this translation for the affirmative "I get in the taxi" but then wrong for the negative, as in this example.

If you were talking about a train or bus or ship, "get on" would be a more natural translation. In English, usually we say "get in" for taxis (or cars).

Probably because getting on is used for trains ... for cars or taxis, getting in is considered more appropriate. English is not my mother tongue, but that's what I understood.