Is the SSO module allowed to create users? If so what info is provided in the SAML message? As you probably know you can pass extra data in the SAML message that you can use to either retrieve the right user or create a new user if none can be found. This error tells you that not all the necesarry info was provided to create the user. Double check your microflow that is triggered for this routine and compare it with the data in the SAML response you got.