The "Prompt not found" error when accessing a shared Microsoft 365 Copilot link from an external source typically stems from organizational security boundaries and licensing requirements.
Key Reasons for the Error
Cross-Tenant Restrictions: Microsoft 365 Copilot prompts are generally restricted to the organization (tenant) where they were created to maintain Enterprise Data Protection (EDP).
Links shared from an external company often cannot be validated across different tenant boundaries.
Licensing Requirements: To view or use a shared prompt, you must have an active Microsoft 365 Copilot license. If you are not signed in with a qualifying account, the prompt will appear unavailable.
Prompt Privacy Settings: By default, new prompts are often private to the creator or their specific environment. If the sender hasn't correctly shared it with "Anyone" or if your organization blocks external content, the link will fail.
Browser & Cookie Issues: Copilot requires third-party cookies to be enabled to validate your license and identity. If these are blocked, it may fail to find the prompt associated with your session.
Reference:
https://learn.microsoft.com/en-us/microsoft-365-copilot/extensibility/known-issues