The neural network package, is currently available for Mathematica 10.X. Wolfram has included more Neural Network algorithms in the core language which partly overlaps with the package, both in functionality and in naming so that the package cannot be run under Mathematica 11.X. However, I have a version of the package ready for Mathematica 11.X. which will soon be made available. With more neural network algorithms built in, there is a smaller need of the package but for teaching ans some special uses, it is probably still valuable.

Jonas Sjöberg/developer of the Neural Network package