Hi, Tyler.
Michael is right. We need more information.
From your error message, I can only say that IdP most likely still isn't aware of this SP. Either because you configured TR wrong, it didn't become active, or changes haven't yet made it to IdP's configuration files on disk.
Please do following:
1. May be you changes haven't been applied yet? Try `# service identity restart` first, wait for 5 mins, then `# service idp restart`. Retry your flow again.
2. Always make sure you removed all cookies related to previous unsuccessful SSO attempts with your Gluu instance (clean all cookies if you are not sure)
3. Go to "Trust relationship" page of web UI and make sure TR you created for this SP is in "active" state. If it isn't, please remove it, and try to create it again, while paying attention to error messages in `/opt/gluu/jetty/identity/logs/oxtrust.log`. Share your findings with us. Please check ["Outbound SAML"](https://gluu.org/docs/ce/3.0.1/admin-guide/saml/#outbound-saml-shibboleth) part of our SAML docs, it was updated today. Make sure you are following all steps, including addition of custom RP config.
4. If it's in "active" state, retry your failing flow while checking `/opt/shibboleth-idp/logs/idp-process.log`. Share any related error messages with us.
5. While running the failing flow, please capture it using **SAMLTracer** Firefox plugin and share it too.
6. Please also provide screenshots of your TR's configuration, and metadata of your SP. Are you sure its metadata is correct and pass validation? You can use [this tool](https://www.samltool.com/validate_xml.php) to verify SP's metadata. Please share metadata with us too.