The message you’re seeing from Copilot suggests that it’s primarily designed to summarize content from documents hosted in OneDrive rather than directly from external websites. This behavior is likely due to security and data access controls that limit Copilot’s ability to fetch content directly from the web unless it’s a Microsoft domain or a resource already accessible through OneDrive.
A workaround would be to download or copy the content from Debian.org into a document (e.g., Word or PDF) and then upload it to your OneDrive. From there, you should be able to use Copilot in PowerPoint to generate a summary based on the document you uploaded. This extra step allows you to bypass the direct web access limitation and still generate summaries for external content.