As stated above, Treadmill is a jailbreak tweak, so you need to have a jailbroken device to use it. The tweak is only compatible with iOS 9.
If you want to install Treadmill, you’ll need to add Ryan Petrich’s beta repository in Cydia:
- Open Cydia
- Go to Sources > Edit > Add
- Then type http://rpetri.ch/repo/ in the popup dialogue box