No such module 'Cocoa' in Swift Playground

This answer helped me:

https://forums.bignerdranch.com/t/import-cocoa-in-playground-is-giving-error/8091/2

I fixed it… never replace an iOS playground by a OS X playground when you name them the same… it’ll stick with the iOS mode…

You are using an iOS playground (UIKit-based), not an OS X playground (Cocoa-based). Try creating a new playground and choosing "OS X" as the type instead of "iOS". It should work fine after that.

You can also change the type for an existing playground in the File Inspector (View→Inspectors→Show File Inspector) under Playground Settings→Platform.

By default, new iOS playgrounds are created with boilerplate including import UIKit; OS X playgrounds are created with boilerplate including import Cocoa, so if you find yourself manually typing "import Cocoa", it's probably a clue you've got the wrong type.

If you are getting this or a similar type of error despite having the playground set properly, try the following:

Close XCode and from the terminal run:

 rm -rf "$(getconf DARWIN_USER_CACHE_DIR)/org.llvm.clang/ModuleCache"

Then reopen XCode and most times this fixes many Playground issues. I run this command quite frequently during an intensive Playground work session (sometimes as many as a few times per hour) and have found liberal use of removing the ModuleCache to solve a multitude of strange playground behaviors.

If you're still having issues with playgrounds you can try:

rm -rf ~/Library/Developer/Xcode/DerivedData

and/or

rm -rf ~/Library/Caches/com.apple.dt.Xcode