XCode 9: UserDefaults not working

I am developing an app and started to implement UserDefaults. It wouldn't work so I tested my implementation in another app. It worked.

I have since, deleted the app from the simulator, restarted my computer, etc. It still doesn't work.

This code works in one app and not the other:

UserDefaults.standard.set("TEST", forKey: "test") 
//does not work 

let test = "TEST" 
UserDefaults.standard.set(test, forKey: "test") 
//does not work 

UserDefaults.standard.set(test, forKey: "test") 
UserDefaults.standard.synchronize() 
//synchronize is legacy and not needed 
//also does not work

Thanks so much for any help.