What a level I have to attain to use this extension?

[hediet]

What a level I have to attain to use this extension?

[hediet]

I'm sorry you had a frustrating experience with this extension!

I don't think this extension is helpful for learning data structures in C yet - I guess converting your data structures to JSON is in C equally challenging than mastering the data structures themselves.

However, the README clearly states the requirements of this extension:

The currently watched expression should evaluate to a JSON Object string, matching the schema of one of the supported visualizers.

You can only visualize expressions that evaluate to a string that represents a JSON object.
If your expression does not evaluate to a string or to a string that is not valid JSON, this extension cannot help you.
You can see to what a expression evaluates to by using the Debug Console.

In case of TypeScript, there is much better support and a lot of values are converted to JSON for you.
In case of C, there is no such support yet.