The Identity Provider with whom you are enabling SSO needs to provide you a metadata containing required fields such as Entity ID, Name ID format, Single Sign-On Service Location URL, Single Sign-On Service Binding and so on.
Apart from this information, Identity Provider also needs to provide their certificate. They can provide it separately in which case, you can follow Guided Configuration and upload that certificate separately.
But they can also provide the certificate within the metadata document. In that case, you can paste the metadata file as it is.
In your case, it looks like IDP has provided you the metadata information but not the certificate.
Please follow this link: https://help.salesforce.com/articleView?id=mc_overview_create_a_key.htm&type=5