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You are trying to pass a non-static member function to server.on("/", handleRoot); which expects a callable of signature void() (i.e. a callable that can be called without any arguments and returning void).

A non-static member function however is not a function callable without any argument. It takes at least one argument, the implicit this pointer to the class instance.

If you want the handler to call handleRoot on the current class instance on which startRoutes was called, then use a lambda to call the member and capture this: