There could be a few different reasons why you're seeing this error:
The user has not refreshed the page since Upscope was installed. Ask the user to refresh and try again.
The Upscope code is not on that particular page, in which case you'll just need to add it and try again. You can check whether Upscope is installed on a page by adding ?upscopeDebug=showAlerts to the url (e.g. https://upscope.io/?upscopeDebug=showAlerts)