Just print the Dictionary to the Console and click on the Test button:

E.g in C#:

foreach(var el in ELEMENTS)
  Console.WriteLine(el);

You seem to have outdated sample tests. You can either:

• Reset the trainer (removes your code, so save it)
• Ignore the sample tests and just hit "attempt"

I'm pretty sure you missed this line from the description:

...Finally, the helper object ELEMENTS has been provided, which is a map from each element symbol to its corresponding full name (e.g. ELEMENTS['Na'] === 'Sodium').

somewhere under the hood platform check something that is absent in the existing tests...

Because the tests expect you to handle it using the given ELEMENTS object. And of course, in sample tests the author can't add all elements as examples. That's why you can't make your own object based on what is shown in the sample tests.

This kata does not require you to actually know the periodic table and all its elements. They are provided for you. You can solve this without knowing a single element's name, theoretically.

Any nitpicks about scientific accuracy would pass by 99.9% of this platform's audience.