ankomonkey -- If the reason you were unable to use it for your chosen hotel is that the country was closed to American (i.e., U.S.) travelers, as most if not all of the EU is, I would think you have a compelling argument that Marriott should extend the validity at least until the ban on US travel is lifted, and a reasonable time thereafter. Of course we're not sure when the EU will lift its ban, but if Marriott doesn't extend the certificates on their own motion, I'd certainly contact them with such a request this autumn.