If you have purchased Dark Noise from the App Store on one device but are not able to download the app from the App Store on a separate iPhone, iPad, or Mac you can try a few things to solve the issue.
- Ensure you are logged into the same iCloud account on the new device as you used to purchase the app originally.
- Restart your device (annoying I know but I have heard from some folks that this has fixed the issue).
- Wait. Unfortunately the App Store seems to sometimes have issues with syncing purchases. After a few hours (sometimes even a couple days) these issues usually resolve themselves.