That's because the email address already exist on the system. You'll have to create a new account, and that means using a new email address.
To provide all permissions to the app, you can also go to:
- Android settings.
Also, on the same Android settings, and then Apps menu,
- Tap the gear icon on the top right.
- Tap on Draw over other apps on the Advanced list.
- Then tap on Prey.
- Enable Permit drawing over other apps.
That should do the trick.