To build a federation trust between AD FS and another application, we need to exchange metadata. Depending on the application concerned, this might be exchanged through an online endpoint or manual exchange of data via an XML document. If you're adding
an RP, you'll need to know the URL for the published metadata or ask the RP to provide you with the XML document so you can manually import it. If you're getting the above error it means that's not the published metadata URL of the RP or you don't have access
to it (e.g. AD FS cannot reach it).