If you need more info about various visibilities in Java
https://iq.opengenus.org/public-private-protected-in-java/

And that's definitely why this Kata is marked as 6kyu

The Kata was designed for Java and was translated by community to Kotlin, that's why some constraints seem illogic when coded in Kotlin.

Already written :
2) The encode method should return an empty String if a null parameter is passed.

Not sure I know the solution then, each test case has its own failing message, or maybe it's a missuse on my own ? Maybe someone can fix it for me ?

It is not an issue to me. Just follow the subject.

If you want to know the testcode, make a print in your method.

Use some try catch or enhanced tests to find out your error.

Use some try catch or enhanced tests to find out how to fix your code.

Help yourself :)=
Pointless ? It allows you to finish the codewar, there is some point at it.

The users have to follow the design, no matter if they understand it or not.

Test cases are explicitely not using assertEquals to not show the test cases easely.

Unknown error is not thrown by any of my test cases.

Increasing the dictionary length discouraged a lot of people doing the exercise, this was already a previous change, this also encourage different solutions.

2. The encode method should return an empty String if a null parameter is passed. (this is taken from description)

This is a codewars issue.

Use a try catch in your own tests "Unknown Error" text is not sent by any of my test cases.

Use a try catch in your own tests "Unknown Error" text is not sent by any of my test cases.

The objective is not to give the solution right away. I will not change this. Tests are part of the work expected.