It's true, you cannot do c.ToUpper(), but you can do Char.ToUpper(c). See https://learn.microsoft.com/en-us/dotnet/api/system.char?view=net-8.0#methods to see what's available with char datatype. Also, be vary of typos, all methods in C# start with uppercase.

Hi. First of all, you should mention which language you're working with, and probably add your code too. Because I already solved this kata in C#, I can see your code, so... The problem is that string in C# is immutable. You need to create a new modified string. Also, the function signature expects you to return a new string, but it doesn't look like you returned anything.

Not that one, maybe it's using System.Linq the one you missed.

Maybe you're missing some using there?