Metal is the standard graphics API for Apple platforms. It’s optimized for Apple devices and thus provides a low-overhead API for the platforms it supports.
Unity uses Metal graphics for Apple platforms by default so there is no need to perform steps to enable it.
Important: Apple deprecated support for OpenGL and OpenGL ES, and you can’t submit applications that use OpenGL/OpenGL ES to the App Store. You should only use Metal to support Unity applications that run on Apple platforms.