If I recall correctly, you need to go to the app's info page in Settings > Apps > All apps > Nova, then tap the 3 dots in the upper right hand corner and tap the option to allow access to restricted settings (not sure if that's the right wording, I forget).
If you're using a secondary profile, not owner, it looks like there might be what's likely an upstream bug that's causing that option not to show up. To get around that, you can install the app in the owner profile, use the app so it asks for that permission, then finally allow it to access restricted settings. After you've done that, the option should show up in the secondary user.