Kata

It might be redundant information to some, but I would add to the description:

Each number in the set has to be used exactly once

The current test setup is incorrect: test.describe blocks are not terminated by test.it blocks, which causes formatting issues and the inability to display runtime.

Duplicate of https://www.codewars.com/kata/587a45b954a6780e37000176

However, your kata has much larger input arrays, which justifies it.

One of the assertion groups:

Random big tests (50 tests with n < 10**5)

I passed 71 tests before hitting time out. You sure it's 50?

Function name is wrong (initially pairs but expects make_pairs)

I'm guessing the =6 is a typo here? otherwise I'm not understanding what you're trying to communicate.

It contains all numbers from 1 to 2*n = 6 once

It should be stated what range of n values will be tested.

This is more of a suggestion, but the current error messages are fairly long to the point where running with an incorrect solution often hits the max buffer size. I'd recommend editing those to provide more succinct errors (e.g. instead of listing the entire set of incorrect values, just display the first invalid case).

Edit: I think I get the =6, as in, for the provided example 2n is 6. Hopefully that's more obvious to others and I was just overlooking it on first read.