Just hit my head against the wall for this. What was actually required to complete the signingCert and related fields is just the metadataUrl in my case
Thank you for sharing this information with the Community!
As this topic can still be valuable to other members, I will share some documentations with useful information on the matter:
the signingCert is used to sign SAML responses/assertions in order to establish trust between the parties and verify the authenticity of the exchange. It is a key component of a SAML integration and I recommend reading through our general doc on Sign and Encrypt SAML Requests for more details;
the Signing Certificate needs to be obtained from the Identity Provider’s side and correctly formatted, usually as a PEM, more information can also be found in this Support Article;