The system is seemingly running fine, yet a recommended First Aid repair failed. What's my next step? I Googled this deeply and couldn't find a current, relevant pointer. Any tips appreciated!
You are pretty light on details but generally speaking when this happens you should boot into recovery mode (Command R) when booting the Mac and go into Disk Utility and try to repair from there.
Failing that you may have to go with a 3rd-party disk repair tool. My go-to is DiskWarrior, there are others that work well too.