This is difficult to identify without more detail, but it's typically an indication of expired or bad API credentials for an app you are attempting to deprovision users from. To verify, navigate to the app from your Okta admin console and test/re-authenticate the API credentials.
If you have additional questions, or need further investigation, I would suggest opening a ticket with Okta support, in order to more effectively track and document this issue.
We look forward to hearing from you!
Justin M. Bergez Technical Support Engineer - Tier 2