Is the endpoint subscribed to the Patching Support site?
Have you enabled the Analysis - Repository Configuration - Oracle Linux in the Patching Support site?
When looking at the registered repositories for the endpoint in the OEL Custom Repository Management dashboard in Patching Support site, does the custom repo you created show up under the endpoint?
If they all match up and applying a Relevant Fixlet still fails, I’d suggest opening a PMR at this point and collecting logs.