It's written in maths, not C#, so it expands to 1 <= n AND n <= 2^64-1. ( Python understands the a <= b <= c syntax like that, but lots of other languages require the AND. )

Which part of that do you not understand? ( Yes, it helps to be able to read that, so this is not a fatuous question. )

ETA: Note that you can Google that exact phrase, and Google will explain it a bit, and offer a link to QuickMath whch will try to explain in more ways and different terms.

But you do, don't you? It's in the description.

They should tell you what input you failed on, unless there's a language without assertion messages that I missed.

It should mostly work. There's another thing to consider, but the test results will probably tell you that.

The tests are correct!

A "power of 2" means 2^n for any n ( where n >= 0 ), so 1, 2, 4, 8, 16, ... See https://en.wikipedia.org/wiki/Exponentiation

If the input can be written as the sum of exactly three of these numbers, return true. If it cannot, return false.

Does this help?

example: 9 -> 2^2 + 2^2 + 2^0
It can be written as three 2^n then its true