If this is C# code then
-----------------------------------------
Correct code
------------------
private string Getstring(string inStr)
{
string input=inStr;
string output =input.substring(0,10);
return output;
}
here is your mistake in bold
function Getstring(string input)
{
string input="abcdefghijklmnopqrstuvwxyz";
output.substring(0,10);
return output;
}