It sounds like your authentication between Project-GC and Geocaching.com expired with an unlucky timing.
The design from Groundspeak is that it lasts for 90 days. Project-GC checks that it's still valid at least every 8th hour. But a job like discovering trackables depends on it existing and being active. My guess is that it expired between Project-GC's last test and that you added the trackable job (or in the middle of the job).
The solution is to log out from Project-GC and authenticate again. Or wait until Project-GC throws you out because it realize the token isn't valid anymore, which should then happen within 8 hours.