There seems to be some issue with browser server communication:

I emptied the test and code areas, I added some unambiguously correct code (JS) and all the time get same error when trying to run sample tests:

Test Results:
Missing language

Server Execution Error:
Missing language

Request Error:
Request failed with status code 422
This error was caused due to an issue processing the web request, not because of an issue executing your code. You can retry the request.

really cool answer, although it requires knowing maths rather than coding

but it works within the parameters set by this exercise