for some reason even though Enhanced Input Subsystem is supposed to be a continual feature starting in 5.1 going forward, and is fully accessible from blueprints (without needing to include it) in order to use it in C++ it must still be explicitly enabled in the Build.cs
[project directory]/Source/[ProjectName]/[ProjectName].Build.cs
PublicDependencyModuleNames.AddRange(new string[] { ..."EnhancedInput"...});
then launch the editor (probably through the .uproject as trying to include a file for a non-included module will have errors in your code.
Tools->“Refresh Visual Studio Project”
if the system is enabled by default in blueprints without any plugin then it should be included by default for use in C++ (if the project was made as a C++ project from start both EnhancedInput and HeadMountedDisplay are included by default)
maybe there is some intention in the future to fold EnhancedInput into InputCore when the older method is fully deprecated, but if the intention that EnhandedInput is the focus going forward then it should probably be included even in blueprint projects.