SAMLResponse is part of SAML2P response message. Thinktecture IdentityServer does not have this protocol implemented. Please see this issue.
If using SAML2P is your hard requirement then I would suggest to try different IAM like Shiboleth or try to implement the feature to IdentityServer yourself. There are .net libraries out there that implement SAML2P. You could look at OIOSAML.NET or ComponentPro UltimateSAML