Where did you download this app from? If it was not the Apple App Store did you load it from a web site? If so, you likely have loaded malware as well as whatever you thought you were loading. Apple’s one, and only, supported site for apps is their own App Store.
Loading anything from a web site is a roll of the dice as to what you actually ended up installing. Loading apps from random web sites basically grants the installer root access to your device, so it can install and modify the device however it wishes to. The practice is known as side loading and is not supported by Apple unless it is from a source you know and trust (such as a company development site deploying proprietary corporate software for employees).
If you downloaded this from anything but the Apple App Store, erase your device and set it up as a new device. Do not restore from a backup if that backup was made since you installed the coupon app. If you have a backup in iCloud or iTunes or finder that was made before you installed anything from the web, you could use that to restore the device.