How to prevent Internet access in NetGuard, "Allow" checkboxes greyed out

As you can see, Allow Wi-Fi when screen is on and Allow mobile when screen is on are greyed out for some reason[.] I don't know whether it is normal or not.

They are greyed out because you choose them to be so. Under Settings, you would be having the following highlighted entries enabled which acts as global default for every app.

_{(Click image to enlarge)}

[Unchecking] Apply rules and conditions does not prevent Internet access to the app.

Because it doesn't work like that. Since you're currently using NetGuard in blacklist mode, in order to prevent an app from accessing Internet, tap on the network icons next to the app's label.

• Tap on the Wi-Fi icon to prevent app from accessing Internet over Wi-Fi


• Tap on mobile data/network icon to prevent app from accessing Internet over mobile data

Once you tap on a network icon its color would change and the corresponding but previously greyed entry would be made available for change as well.

_{(Image courtesy of Marcel Bokhorst at his GitHub)}

You can use official help for more information on NetGuard's usage.