Since conan-io/conan-center-index maintains a recipe packaging stevengj/nlopt library, this recipe will not be maintained anymore. If possible, please use the package nlopt instead.
Add nlopt/2.4.2@vthiery/stable in the list of requirements of your conanfile. See how to use a conanfile.py for more information.
See Conan-Package-Tools Github page for more information on packaging.