• Examples are not specifications
• No random tests
• There're already katas about repeating a string many times, and writing extension methods is not new either.

changed

In C#.
I said it will be better with some examples because this kata appears to be for beginners.

Which language? Original in JS has examples

It will be better if some examples were given

As other people have said, change "Boolean" to "bool".
Other than that, the kata is ready and should be for 8 kyu.