As of 2024, i am getting this error, for example when launching a React Native application (i.e. yarn ios) and the Simulator device is already booted. You can just click "Ok" to ignore it and the app launches nevertheless, but it still bugged me. Other answers suggest to disable the feature in Simulator which just wakes up a recently used device instead of booting it. This removes the error popup but it causes your Simulator device to boot every time you start Simulator, which actually costs more time than just ignoring the popup. What worked for me was to actually to just create a new Simulator in the Simulator app via File->New Simulator... and then launching my React Native app using that one. What i also did was (it is probably optional) delete all Simulators before creating the new one so i just end up with 1 device xcrun simctl delete all. Also, i selected iOS 18 for my new Simulator (my previous one was iOS 17)