Why I get error on string concatination? exit status 1
invalid operands of types 'const char [5]' and 'const char [2]' to binary 'operator+'

  String message = "Page not found\n\n";
  message += "URI: " + HTTP.uri() + "\n";
  message += "Method: " + (HTTP.method() == HTTP_GET)? "GET": "POST" + "\n";
  message += "Arguments: " + HTTP.args() + "\n";
The operators of the class String apply only for an instance of String. In your expression you concatenate two c-strings "POST" + "\n"