There are several ways that code can be debugged within the challenge IDE.
Most compilation & runtime errors should show up automatically in the results pane.
Write your own test cases!
The Sample Tests editor is available for developers to write their own test cases. These are their own cases, and usually they will be provided with a few basic examples. Whatever changes they make to their code will be provided to the reviewer, so you they use this as an extra way to stand out, showing their ability to write quality, testable code.
STDOUT / Console Output
While Qualified doesn't support interactive debugging, developers can write to
stdout (sometimes called "system out" or "console") in any language for outputting debugging information. When they run their code, this will show alongside the test output.