Quiconque and n'importe qui are pretty much synonyms but belong to a different register. The register of a language distinguishes at whom the sentence is addressed, the level of formality employed and under which circumstances it was made. It determines choice of syntax and vocabulary.

There are six registers:

1) Literary language (le registre soutenu) : in poetry and high literature
2) Formal language (le registre formel) : at formal occasions
3) Normal language (le registre normal) : normal everyday language
4) Familiar language (le registre familier) : among friends and family
5) Lower grade popular language (le registre populaire): among buddies
6) Slang (le registre vulgaire): vulgar language

"N'importe qui" I would place in "le registre normal". "Quiconque" belongs to a higher register, either formal or literary French. A native speaker would be able to shed more light on this.

-- Chris (not a native speaker).

There is something akin to this in every language, by the way. In English you could say, Sir, Mister, buddy or bloke to refer to a specific male. In this sense they are all synonyms but each belongs to its own register. -- Chris.

Thank you Chris, that was really useful. I asked this question because while I was taking the tests on kwiziq, I tried using "Quiconque" in place of "n'importe qui" and was marked wrong.

Since the question is about practicing the use of "n'importe qui" you are supposed to use that phrase. -- Chris.