I, too, am experiencing this.
I am in customer mode - not MSP mode - and the Captive Portal profile/page is being used successfully in a guest wireless network defined in the customer's group. All I need to do is update the Terms & Conditions text, but the "Scope not found" error presents itself.
Is there a different location where I must perform this? What is causing the error, so I can know what to avoid in the future?