Copying frameworks into the system frameworks folder is a bad idea. Just put SDL2.framework in the same folder as your .xcodeproj, link to it in Xcode (hit the + and then hit Add Other..., then navigate to SDL2.framework), and Xcode will automatically set it to be copied to your application bundle when your application is built. If Xcode can’t find it when building your app, go into your project’s build settings and add the folder where SDL2.framework is located to Framework Search Path (if it’s in the same folder as your .xcodeproj it should find it, but 
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Or put SDL2.framework somewhere in your user folder, link to it the same way mentioned above, and make sure your project’s build settings have that folder in the Framework Search Path.
Also, use the Finder to copy/move frameworks, since they’re special folders with symlinks inside pointing to other files/folders inside the framework that need to be preserved. In the past I’ve used the command line to copy them and they break.