Because your Lookout payment is controlled by Apple, you will need to cancel your subscription through your iTunes account:
If you are currently on iOS 11:
1. Open the App Store on your device
2. Tap your Apple ID Profile Picture at the top
3. Sign into iTunes
4. Tap Subscriptions
5. Select Lookout
6. Turn Auto-Renewal OFF
If you are currently on iOS 10 or below:
1. Open the App Store on your device
2. Scroll to bottom and tap your Apple ID
3. Sign into iTunes
4. Under Subscriptions, tap Manage
5. Select Lookout
6. Turn Auto-Renewal OFF