Seems that you have configured your idp manually, not by using the discovery URL, which whould have inserted all URLs/endpoints automatically.
Now, open the “OpenID Connect metatdata document” in a browser, look for end_session_endpoint and enter this URL into the “Logout URL” field.
Once you are here, also enter the issuer and the userinfo_endpoint from the document in your Keycloak idp config and enable “Validate Signatures”.
This might also solve your other issue.