It sounds like you’re calling ISamlIdentityProvider.ReceiveSsoAsync instead of ISamlServiceProvider.ReceiveSsoAsync.
Also, the code you included is for an identity provider but you’re acting as the service provider.
I suggest taking a look at the ExampleServiceProvider project under the Examples\SSO folder. The SamlController demonstrates calling ISamlServiceProvider.InitiateSsoAsync to create and send a SAML authn request to the specified partner identity provider. It calls ISamlServiceProvider.ReceiveSsoAsync to receive and process the SAML response.