"getLengthOfMissingArray" is the name of function you have to write, so the error message is pointing to code that is written by you.

Somewhere in your code, you are trying to access .length on something that is null (we can only guess where it comes from, without seeing your code).

Make use of console.logs to see where this happens.

In which language?

the case of letters is ignored.

Your code fails with this test for example:

Assert::That(is_isogram("moOse"), Equals(false));