Could be usefull to make more clear description.

Can you be more specific besides dumping a whole Wikipedia article which, again, doesn't say much about the topic at hand?

The argument list in the function (p, n, x) is incorrect

(p, x, n) is the actual order

modulus 'n' is the message 'x' and vice versa

All the test cases pass once you replace them. This should be fixed, else it's a nice nice kata

"Your Solution" lists the arguments in the wrong order; it should be function decrypt(p, n, x) {}.

The order of arguments in the starting code and your expected solution is different. p, n, x is expected, p, x, n is available at the start.
Leading to confusion on error like the issue below this.

props for the test cases matt c. you're too kind

I've added some random test cases to the kata, hope it helps! Please mark this issue as resolved if it is thanks! :)

Nice work, but it needs a lot more test cases (preferably some random ones).

Otherwise it's easy to cheat by hardcoding the answer (see my solution).