This comes from the United Nation Secretary General qualifications website:
Q. Who elects the Secretary-General?
A. The Security Council nominates a Secretary-General to the General Assembly which then votes to appoint the Council's choice. The Security Council is barred from nominating an individual that is from one of Security Council's five permanent members (China, France, Russia, UK, U.S.).
Can you cite this specific nominating restriction? I read through the Charter after this was posted earlier and I can find no such restriction.