An AD FS trust with Microsoft Online already exists. The Microsoft Online identifiers are "urn:federation:MicrosoftOnline" ...

An AD FS trust with Microsoft Online already exists. The Microsoft Online identifiers are "urn:federation:MicrosoftOnline" and "https://login.microsoftonline.com/ext/STS.srf". Each identifier for a relying party trust must be unique across all relying party trusts in AD FS configuration.